ACE Working Group L. Seitz
Internet-Draft RISE
Intended status: Standards Track G. Selander
Expires: September 28, 2019 Ericsson
E. Wahlstroem
S. Erdtman
Spotify AB
H. Tschofenig
Arm Ltd.
March 27, 2019
Authentication and Authorization for Constrained Environments (ACE)
using the OAuth 2.0 Framework (ACE-OAuth)
draft-ietf-ace-oauth-authz-24
Abstract
This specification defines a framework for authentication and
authorization in Internet of Things (IoT) environments called ACE-
OAuth. The framework is based on a set of building blocks including
OAuth 2.0 and CoAP, thus making a well-known and widely used
authorization solution suitable for IoT devices. Existing
specifications are used where possible, but where the constraints of
IoT devices require it, extensions are added and profiles are
defined.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 28, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. OAuth 2.0 . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. CoAP . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Protocol Interactions . . . . . . . . . . . . . . . . . . . . 11
5. Framework . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Discovering Authorization Servers . . . . . . . . . . . . 16
5.1.1. Unauthorized Resource Request Message . . . . . . . . 16
5.1.2. AS Request Creation Hints . . . . . . . . . . . . . . 17
5.1.2.1. The Client-Nonce Parameter . . . . . . . . . . . 19
5.2. Authorization Grants . . . . . . . . . . . . . . . . . . 20
5.3. Client Credentials . . . . . . . . . . . . . . . . . . . 20
5.4. AS Authentication . . . . . . . . . . . . . . . . . . . . 21
5.5. The Authorization Endpoint . . . . . . . . . . . . . . . 21
5.6. The Token Endpoint . . . . . . . . . . . . . . . . . . . 21
5.6.1. Client-to-AS Request . . . . . . . . . . . . . . . . 22
5.6.2. AS-to-Client Response . . . . . . . . . . . . . . . . 25
5.6.3. Error Response . . . . . . . . . . . . . . . . . . . 27
5.6.4. Request and Response Parameters . . . . . . . . . . . 28
5.6.4.1. Grant Type . . . . . . . . . . . . . . . . . . . 28
5.6.4.2. Token Type . . . . . . . . . . . . . . . . . . . 28
5.6.4.3. Profile . . . . . . . . . . . . . . . . . . . . . 29
5.6.4.4. Client-Nonce . . . . . . . . . . . . . . . . . . 29
5.6.5. Mapping Parameters to CBOR . . . . . . . . . . . . . 29
5.7. The Introspection Endpoint . . . . . . . . . . . . . . . 30
5.7.1. Introspection Request . . . . . . . . . . . . . . . . 31
5.7.2. Introspection Response . . . . . . . . . . . . . . . 32
5.7.3. Error Response . . . . . . . . . . . . . . . . . . . 33
5.7.4. Mapping Introspection parameters to CBOR . . . . . . 34
5.8. The Access Token . . . . . . . . . . . . . . . . . . . . 34
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5.8.1. The Authorization Information Endpoint . . . . . . . 35
5.8.1.1. Verifying an Access Token . . . . . . . . . . . . 36
5.8.1.2. Protecting the Authorization Information
Endpoint . . . . . . . . . . . . . . . . . . . . 38
5.8.2. Client Requests to the RS . . . . . . . . . . . . . . 38
5.8.3. Token Expiration . . . . . . . . . . . . . . . . . . 39
5.8.4. Key Expiration . . . . . . . . . . . . . . . . . . . 40
6. Security Considerations . . . . . . . . . . . . . . . . . . . 41
6.1. Unprotected AS Request Creation Hints . . . . . . . . . . 42
6.2. Minimal security requirements for communication . 42
6.3. Use of Nonces for Token Freshness . . . . . . . . . . . . 43
6.4. Combining profiles . . . . . . . . . . . . . . . . . . . 44
6.5. Unprotected Information . . . . . . . . . . . . . . . . . 44
6.6. Identifying audiences . . . . . . . . . . . . . . . . . . 44
6.7. Denial of service against or with Introspection . . 45
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 46
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
8.1. ACE Authorization Server Request Creation Hints . . . . . 46
8.2. OAuth Extensions Error Registration . . . . . . . . . . . 47
8.3. OAuth Error Code CBOR Mappings Registry . . . . . . . . . 47
8.4. OAuth Grant Type CBOR Mappings . . . . . . . . . . . . . 48
8.5. OAuth Access Token Types . . . . . . . . . . . . . . . . 48
8.6. OAuth Access Token Type CBOR Mappings . . . . . . . . . . 49
8.6.1. Initial Registry Contents . . . . . . . . . . . . . . 49
8.7. ACE Profile Registry . . . . . . . . . . . . . . . . . . 49
8.8. OAuth Parameter Registration . . . . . . . . . . . . . . 50
8.9. OAuth Parameters CBOR Mappings Registry . . . . . . . . . 50
8.10. OAuth Introspection Response Parameter Registration . . . 51
8.11. OAuth Token Introspection Response CBOR Mappings Registry 51
8.12. JSON Web Token Claims . . . . . . . . . . . . . . . . . . 51
8.13. CBOR Web Token Claims . . . . . . . . . . . . . . . . . . 52
8.14. Media Type Registrations . . . . . . . . . . . . . . . . 53
8.15. CoAP Content-Format Registry . . . . . . . . . . . . . . 54
8.16. Expert Review Instructions . . . . . . . . . . . . . . . 54
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 55
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 56
10.1. Normative References . . . . . . . . . . . . . . . . . . 56
10.2. Informative References . . . . . . . . . . . . . . . . . 58
Appendix A. Design Justification . . . . . . . . . . . . . . . . 60
Appendix B. Roles and Responsibilities . . . . . . . . . . . . . 64
Appendix C. Requirements on Profiles . . . . . . . . . . . . . . 66
Appendix D. Assumptions on AS knowledge about C and RS . . . . . 67
Appendix E. Deployment Examples . . . . . . . . . . . . . . . . 67
E.1. Local Token Validation . . . . . . . . . . . . . . . . . 67
E.2. Introspection Aided Token Validation . . . . . . . . . . 72
Appendix F. Document Updates . . . . . . . . . . . . . . . . . . 76
F.1. Version -21 to 22 . . . . . . . . . . . . . . . . . . . . 76
F.2. Version -20 to 21 . . . . . . . . . . . . . . . . . . . . 76
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F.3. Version -19 to 20 . . . . . . . . . . . . . . . . . . . . 76
F.4. Version -18 to -19 . . . . . . . . . . . . . . . . . . . 77
F.5. Version -17 to -18 . . . . . . . . . . . . . . . . . . . 77
F.6. Version -16 to -17 . . . . . . . . . . . . . . . . . . . 77
F.7. Version -15 to -16 . . . . . . . . . . . . . . . . . . . 78
F.8. Version -14 to -15 . . . . . . . . . . . . . . . . . . . 78
F.9. Version -13 to -14 . . . . . . . . . . . . . . . . . . . 78
F.10. Version -12 to -13 . . . . . . . . . . . . . . . . . . . 78
F.11. Version -11 to -12 . . . . . . . . . . . . . . . . . . . 78
F.12. Version -10 to -11 . . . . . . . . . . . . . . . . . . . 79
F.13. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 79
F.14. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 79
F.15. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 79
F.16. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 80
F.17. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 80
F.18. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 80
F.19. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 80
F.20. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 80
F.21. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 81
F.22. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 81
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 82
1. Introduction
Authorization is the process for granting approval to an entity to
access a resource [RFC4949]. The authorization task itself can best
be described as granting access to a requesting client, for a
resource hosted on a device, the resource server (RS). This exchange
is mediated by one or multiple authorization servers (AS). Managing
authorization for a large number of devices and users can be a
complex task.
While prior work on authorization solutions for the Web and for the
mobile environment also applies to the Internet of Things (IoT)
environment, many IoT devices are constrained, for example, in terms
of processing capabilities, available memory, etc. For web
applications on constrained nodes, this specification RECOMMENDS the
use of CoAP [RFC7252] as replacement for HTTP.
A detailed treatment of constraints can be found in [RFC7228], and
the different IoT deployments present a continuous range of device
and network capabilities. Taking energy consumption as an example:
At one end there are energy-harvesting or battery powered devices
which have a tight power budget, on the other end there are mains-
powered devices, and all levels in between.
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Hence, IoT devices may be very different in terms of available
processing and message exchange capabilities and there is a need to
support many different authorization use cases [RFC7744].
This specification describes a framework for authentication and
authorization in constrained environments (ACE) built on re-use of
OAuth 2.0 [RFC6749], thereby extending authorization to Internet of
Things devices. This specification contains the necessary building
blocks for adjusting OAuth 2.0 to IoT environments.
More detailed, interoperable specifications can be found in profiles.
Implementations may claim conformance with a specific profile,
whereby implementations utilizing the same profile interoperate while
implementations of different profiles are not expected to be
interoperable. Some devices, such as mobile phones and tablets, may
implement multiple profiles and will therefore be able to interact
with a wider range of low end devices. Requirements on profiles are
described at contextually appropriate places throughout this
specification, and also summarized in Appendix C.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Certain security-related terms such as "authentication",
"authorization", "confidentiality", "(data) integrity", "message
authentication code", and "verify" are taken from [RFC4949].
Since exchanges in this specification are described as RESTful
protocol interactions, HTTP [RFC7231] offers useful terminology.
Terminology for entities in the architecture is defined in OAuth 2.0
[RFC6749] such as client (C), resource server (RS), and authorization
server (AS).
Note that the term "endpoint" is used here following its OAuth
definition, which is to denote resources such as token and
introspection at the AS and authz-info at the RS (see Section 5.8.1
for a definition of the authz-info endpoint). The CoAP [RFC7252]
definition, which is "An entity participating in the CoAP protocol"
is not used in this specification.
The specifications in this document is called the "framework" or "ACE
framework". When referring to "profiles of this framework" it refers
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to additional specifications that define the use of this
specification with concrete transport, and communication security
protocols (e.g., CoAP over DTLS).
We use the term "Access Information" for parameters other than the
access token provided to the client by the AS to enable it to access
the RS (e.g. public key of the RS, profile supported by RS).
We use the term "Authorization Information" to denote all
information, including the claims of relevant access tokens, that an
RS uses to determine whether an access request should be granted.
3. Overview
This specification defines the ACE framework for authorization in the
Internet of Things environment. It consists of a set of building
blocks.
The basic block is the OAuth 2.0 [RFC6749] framework, which enjoys
widespread deployment. Many IoT devices can support OAuth 2.0
without any additional extensions, but for certain constrained
settings additional profiling is needed.
Another building block is the lightweight web transfer protocol CoAP
[RFC7252], for those communication environments where HTTP is not
appropriate. CoAP typically runs on top of UDP, which further
reduces overhead and message exchanges. While this specification
defines extensions for the use of OAuth over CoAP, other underlying
protocols are not prohibited from being supported in the future, such
as HTTP/2, MQTT, BLE and QUIC.
A third building block is CBOR [RFC7049], for encodings where JSON
[RFC8259] is not sufficiently compact. CBOR is a binary encoding
designed for small code and message size, which may be used for
encoding of self contained tokens, and also for encoding payload
transferred in protocol messages.
A fourth building block is the compact CBOR-based secure message
format COSE [RFC8152], which enables application layer security as an
alternative or complement to transport layer security (DTLS [RFC6347]
or TLS [RFC8446]). COSE is used to secure self-contained tokens such
as proof-of-possession (PoP) tokens, which is an extension to the
OAuth tokens. The default token format is defined in CBOR web token
(CWT) [RFC8392]. Application layer security for CoAP using COSE can
be provided with OSCORE [I-D.ietf-core-object-security].
With the building blocks listed above, solutions satisfying various
IoT device and network constraints are possible. A list of
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constraints is described in detail in [RFC7228] and a description of
how the building blocks mentioned above relate to the various
constraints can be found in Appendix A.
Luckily, not every IoT device suffers from all constraints. The ACE
framework nevertheless takes all these aspects into account and
allows several different deployment variants to co-exist, rather than
mandating a one-size-fits-all solution. It is important to cover the
wide range of possible interworking use cases and the different
requirements from a security point of view. Once IoT deployments
mature, popular deployment variants will be documented in the form of
ACE profiles.
3.1. OAuth 2.0
The OAuth 2.0 authorization framework enables a client to obtain
scoped access to a resource with the permission of a resource owner.
Authorization information, or references to it, is passed between the
nodes using access tokens. These access tokens are issued to clients
by an authorization server with the approval of the resource owner.
The client uses the access token to access the protected resources
hosted by the resource server.
A number of OAuth 2.0 terms are used within this specification:
The token and introspection Endpoints:
The AS hosts the token endpoint that allows a client to request
access tokens. The client makes a POST request to the token
endpoint on the AS and receives the access token in the response
(if the request was successful).
In some deployments, a token introspection endpoint is provided by
the AS, which can be used by the RS if it needs to request
additional information regarding a received access token. The RS
makes a POST request to the introspection endpoint on the AS and
receives information about the access token in the response. (See
"Introspection" below.)
Access Tokens:
Access tokens are credentials needed to access protected
resources. An access token is a data structure representing
authorization permissions issued by the AS to the client. Access
tokens are generated by the AS and consumed by the RS. The access
token content is opaque to the client.
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Access tokens can have different formats, and various methods of
utilization (e.g., cryptographic properties) based on the security
requirements of the given deployment.
Refresh Tokens:
Refresh tokens are credentials used to obtain access tokens.
Refresh tokens are issued to the client by the authorization
server and are used to obtain a new access token when the current
access token becomes invalid or expires, or to obtain additional
access tokens with identical or narrower scope (access tokens may
have a shorter lifetime and fewer permissions than authorized by
the resource owner). Issuing a refresh token is optional at the
discretion of the authorization server. If the authorization
server issues a refresh token, it is included when issuing an
access token (i.e., step (B) in Figure 1).
A refresh token in OAuth 2.0 is a string representing the
authorization granted to the client by the resource owner. The
string is usually opaque to the client. The token denotes an
identifier used to retrieve the authorization information. Unlike
access tokens, refresh tokens are intended for use only with
authorization servers and are never sent to resource servers. In
this framework, refresh tokens are encoded in binary instead of
strings, if used.
Proof of Possession Tokens:
An access token may be bound to a cryptographic key, which is then
used by an RS to authenticate requests from a client. Such tokens
are called proof-of-possession access tokens (or PoP access
tokens).
The proof-of-possession (PoP) security concept assumes that the AS
acts as a trusted third party that binds keys to access tokens.
These so called PoP keys are then used by the client to
demonstrate the possession of the secret to the RS when accessing
the resource. The RS, when receiving an access token, needs to
verify that the key used by the client matches the one bound to
the access token. When this specification uses the term "access
token" it is assumed to be a PoP access token token unless
specifically stated otherwise.
The key bound to the access token (the PoP key) may use either
symmetric or asymmetric cryptography. The appropriate choice of
the kind of cryptography depends on the constraints of the IoT
devices as well as on the security requirements of the use case.
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Symmetric PoP key:
The AS generates a random symmetric PoP key. The key is either
stored to be returned on introspection calls or encrypted and
included in the access token. The PoP key is also encrypted
for the client and sent together with the access token to the
client.
Asymmetric PoP key:
An asymmetric key pair is generated on the client and the
public key is sent to the AS (if it does not already have
knowledge of the client's public key). Information about the
public key, which is the PoP key in this case, is either stored
to be returned on introspection calls or included inside the
access token and sent back to the requesting client. The RS
can identify the client's public key from the information in
the token, which allows the client to use the corresponding
private key for the proof of possession.
The access token is either a simple reference, or a structured
information object (e.g., CWT [RFC8392]) protected by a
cryptographic wrapper (e.g., COSE [RFC8152]). The choice of PoP
key does not necessarily imply a specific credential type for the
integrity protection of the token.
Scopes and Permissions:
In OAuth 2.0, the client specifies the type of permissions it is
seeking to obtain (via the scope parameter) in the access token
request. In turn, the AS may use the scope response parameter to
inform the client of the scope of the access token issued. As the
client could be a constrained device as well, this specification
defines the use of CBOR encoding as data format, see Section 5, to
request scopes and to be informed what scopes the access token
actually authorizes.
The values of the scope parameter in OAuth 2.0 are expressed as a
list of space-delimited, case-sensitive strings, with a semantic
that is well-known to the AS and the RS. More details about the
concept of scopes is found under Section 3.3 in [RFC6749].
Claims:
Information carried in the access token or returned from
introspection, called claims, is in the form of name-value pairs.
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An access token may, for example, include a claim identifying the
AS that issued the token (via the "iss" claim) and what audience
the access token is intended for (via the "aud" claim). The
audience of an access token can be a specific resource or one or
many resource servers. The resource owner policies influence what
claims are put into the access token by the authorization server.
While the structure and encoding of the access token varies
throughout deployments, a standardized format has been defined
with the JSON Web Token (JWT) [RFC7519] where claims are encoded
as a JSON object. In [RFC8392], an equivalent format using CBOR
encoding (CWT) has been defined.
Introspection:
Introspection is a method for a resource server to query the
authorization server for the active state and content of a
received access token. This is particularly useful in those cases
where the authorization decisions are very dynamic and/or where
the received access token itself is an opaque reference rather
than a self-contained token. More information about introspection
in OAuth 2.0 can be found in [RFC7662].
3.2. CoAP
CoAP is an application layer protocol similar to HTTP, but
specifically designed for constrained environments. CoAP typically
uses datagram-oriented transport, such as UDP, where reordering and
loss of packets can occur. A security solution needs to take the
latter aspects into account.
While HTTP uses headers and query strings to convey additional
information about a request, CoAP encodes such information into
header parameters called 'options'.
CoAP supports application-layer fragmentation of the CoAP payloads
through blockwise transfers [RFC7959]. However, blockwise transfer
does not increase the size limits of CoAP options, therefore data
encoded in options has to be kept small.
Transport layer security for CoAP can be provided by DTLS or TLS
[RFC6347][RFC8446] [I-D.ietf-tls-dtls13]. CoAP defines a number of
proxy operations that require transport layer security to be
terminated at the proxy. One approach for protecting CoAP
communication end-to-end through proxies, and also to support
security for CoAP over a different transport in a uniform way, is to
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provide security at the application layer using an object-based
security mechanism such as COSE [RFC8152].
One application of COSE is OSCORE [I-D.ietf-core-object-security],
which provides end-to-end confidentiality, integrity and replay
protection, and a secure binding between CoAP request and response
messages. In OSCORE, the CoAP messages are wrapped in COSE objects
and sent using CoAP.
This framework RECOMMENDS the use of CoAP as replacement for HTTP for
use in constrained environments.
4. Protocol Interactions
The ACE framework is based on the OAuth 2.0 protocol interactions
using the token endpoint and optionally the introspection endpoint.
A client obtains an access token, and optionally a refresh token,
from an AS using the token endpoint and subsequently presents the
access token to a RS to gain access to a protected resource. In most
deployments the RS can process the access token locally, however in
some cases the RS may present it to the AS via the introspection
endpoint to get fresh information. These interactions are shown in
Figure 1. An overview of various OAuth concepts is provided in
Section 3.1.
The OAuth 2.0 framework defines a number of "protocol flows" via
grant types, which have been extended further with extensions to
OAuth 2.0 (such as [RFC7521] and [I-D.ietf-oauth-device-flow]). What
grant types works best depends on the usage scenario and [RFC7744]
describes many different IoT use cases but there are two preferred
grant types, namely the Authorization Code Grant (described in
Section 4.1 of [RFC7521]) and the Client Credentials Grant (described
in Section 4.4 of [RFC7521]). The Authorization Code Grant is a good
fit for use with apps running on smart phones and tablets that
request access to IoT devices, a common scenario in the smart home
environment, where users need to go through an authentication and
authorization phase (at least during the initial setup phase). The
native apps guidelines described in [RFC8252] are applicable to this
use case. The Client Credential Grant is a good fit for use with IoT
devices where the OAuth client itself is constrained. In such a
case, the resource owner has pre-arranged access rights for the
client with the authorization server, which is often accomplished
using a commissioning tool.
The consent of the resource owner, for giving a client access to a
protected resource, can be provided dynamically as in the traditional
OAuth flows, or it could be pre-configured by the resource owner as
authorization policies at the AS, which the AS evaluates when a token
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request arrives. The resource owner and the requesting party (i.e.,
client owner) are not shown in Figure 1.
This framework supports a wide variety of communication security
mechanisms between the ACE entities, such as client, AS, and RS. It
is assumed that the client has been registered (also called enrolled
or onboarded) to an AS using a mechanism defined outside the scope of
this document. In practice, various techniques for onboarding have
been used, such as factory-based provisioning or the use of
commissioning tools. Regardless of the onboarding technique, this
provisioning procedure implies that the client and the AS exchange
credentials and configuration parameters. These credentials are used
to mutually authenticate each other and to protect messages exchanged
between the client and the AS.
It is also assumed that the RS has been registered with the AS,
potentially in a similar way as the client has been registered with
the AS. Established keying material between the AS and the RS allows
the AS to apply cryptographic protection to the access token to
ensure that its content cannot be modified, and if needed, that the
content is confidentiality protected.
The keying material necessary for establishing communication security
between C and RS is dynamically established as part of the protocol
described in this document.
At the start of the protocol, there is an optional discovery step
where the client discovers the resource server and the resources this
server hosts. In this step, the client might also determine what
permissions are needed to access the protected resource. A generic
procedure is described in Section 5.1, profiles MAY define other
procedures for discovery.
In Bluetooth Low Energy, for example, advertisements are broadcasted
by a peripheral, including information about the primary services.
In CoAP, as a second example, a client can make a request to "/.well-
known/core" to obtain information about available resources, which
are returned in a standardized format as described in [RFC6690].
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+--------+ +---------------+
| |---(A)-- Token Request ------->| |
| | | Authorization |
| |<--(B)-- Access Token ---------| Server |
| | + Access Information | |
| | + Refresh Token (optional) +---------------+
| | ^ |
| | Introspection Request (D)| |
| Client | (optional) | |
| | Response | |(E)
| | (optional) | v
| | +--------------+
| |---(C)-- Token + Request ----->| |
| | | Resource |
| |<--(F)-- Protected Resource ---| Server |
| | | |
+--------+ +--------------+
Figure 1: Basic Protocol Flow.
Requesting an Access Token (A):
The client makes an access token request to the token endpoint at
the AS. This framework assumes the use of PoP access tokens (see
Section 3.1 for a short description) wherein the AS binds a key to
an access token. The client may include permissions it seeks to
obtain, and information about the credentials it wants to use
(e.g., symmetric/asymmetric cryptography or a reference to a
specific credential).
Access Token Response (B):
If the AS successfully processes the request from the client, it
returns an access token and optionally a refresh token (note that
only certain grant types support refresh tokens). It can also
return additional parameters, referred to as "Access Information".
In addition to the response parameters defined by OAuth 2.0 and
the PoP access token extension, this framework defines parameters
that can be used to inform the client about capabilities of the
RS. More information about these parameters can be found in
Section 5.6.4.
Resource Request (C):
The client interacts with the RS to request access to the
protected resource and provides the access token. The protocol to
use between the client and the RS is not restricted to CoAP.
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HTTP, HTTP/2, QUIC, MQTT, Bluetooth Low Energy, etc., are also
viable candidates.
Depending on the device limitations and the selected protocol,
this exchange may be split up into two parts:
(1) the client sends the access token containing, or
referencing, the authorization information to the RS, that may
be used for subsequent resource requests by the client, and
(2) the client makes the resource access request, using the
communication security protocol and other Access Information
obtained from the AS.
The Client and the RS mutually authenticate using the security
protocol specified in the profile (see step B) and the keys
obtained in the access token or the Access Information. The RS
verifies that the token is integrity protected by the AS and
compares the claims contained in the access token with the
resource request. If the RS is online, validation can be handed
over to the AS using token introspection (see messages D and E)
over HTTP or CoAP.
Token Introspection Request (D):
A resource server may be configured to introspect the access token
by including it in a request to the introspection endpoint at that
AS. Token introspection over CoAP is defined in Section 5.7 and
for HTTP in [RFC7662].
Note that token introspection is an optional step and can be
omitted if the token is self-contained and the resource server is
prepared to perform the token validation on its own.
Token Introspection Response (E):
The AS validates the token and returns the most recent parameters,
such as scope, audience, validity etc. associated with it back to
the RS. The RS then uses the received parameters to process the
request to either accept or to deny it.
Protected Resource (F):
If the request from the client is authorized, the RS fulfills the
request and returns a response with the appropriate response code.
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The RS uses the dynamically established keys to protect the
response, according to used communication security protocol.
5. Framework
The following sections detail the profiling and extensions of OAuth
2.0 for constrained environments, which constitutes the ACE
framework.
Credential Provisioning
For IoT, it cannot be assumed that the client and RS are part of a
common key infrastructure, so the AS provisions credentials or
associated information to allow mutual authentication. These
credentials need to be provided to the parties before or during
the authentication protocol is executed, and may be re-used for
subsequent token requests.
Proof-of-Possession
The ACE framework, by default, implements proof-of-possession for
access tokens, i.e., that the token holder can prove being a
holder of the key bound to the token. The binding is provided by
the "cnf" claim [I-D.ietf-ace-cwt-proof-of-possession] indicating
what key is used for proof-of-possession. If a client needs to
submit a new access token, e.g., to obtain additional access
rights, they can request that the AS binds this token to the same
key as the previous one.
ACE Profiles
The client or RS may be limited in the encodings or protocols it
supports. To support a variety of different deployment settings,
specific interactions between client and RS are defined in an ACE
profile. In ACE framework the AS is expected to manage the
matching of compatible profile choices between a client and an RS.
The AS informs the client of the selected profile using the
"profile" parameter in the token response.
OAuth 2.0 requires the use of TLS both to protect the communication
between AS and client when requesting an access token; between client
and RS when accessing a resource and between AS and RS if
introspection is used. In constrained settings TLS is not always
feasible, or desirable. Nevertheless it is REQUIRED that the data
exchanged with the AS is encrypted, integrity protected and protected
against message replay. It is also REQUIRED that the AS and the
endpoint communicating with it (client or RS) perform mutual
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authentication. Furthermore it MUST be assured that responses are
bound to the requests in the sense that the receiver of a response
can be certain that the response actually belongs to a certain
request.
Profiles MUST specify a communication security protocol that provides
the features required above.
In OAuth 2.0 the communication with the Token and the Introspection
endpoints at the AS is assumed to be via HTTP and may use Uri-query
parameters. When profiles of this framework use CoAP instead, this
framework REQUIRES the use of the following alternative instead of
Uri-query parameters: The sender (client or RS) encodes the
parameters of its request as a CBOR map and submits that map as the
payload of the POST request. Profiles that use CBOR encoding of
protocol message parameters MUST use the media format 'application/
ace+cbor', unless the protocol message is wrapped in another Content-
Format (e.g. object security). If CoAP is used for communication,
the Content-Format MUST be abbreviated with the ID: 19 (see
Section 8.15).
The OAuth 2.0 AS uses a JSON structure in the payload of its
responses both to client and RS. If CoAP is used, this framework
REQUIRES the use of CBOR [RFC7049] instead of JSON. Depending on the
profile, the CBOR payload MAY be enclosed in a non-CBOR cryptographic
wrapper.
5.1. Discovering Authorization Servers
In order to determine the AS in charge of a resource hosted at the
RS, C MAY send an initial Unauthorized Resource Request message to
RS. RS then denies the request and sends the address of its AS back
to C.
Instead of the initial Unauthorized Resource Request message, other
discovery methods may be used, or the client may be pre-provisioned
with the address of the AS.
5.1.1. Unauthorized Resource Request Message
The optional Unauthorized Resource Request message is a request for a
resource hosted by RS for which no proper authorization is granted.
RS MUST treat any request for a protected resource as Unauthorized
Resource Request message when any of the following holds:
o The request has been received on an unprotected channel.
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o RS has no valid access token for the sender of the request
regarding the requested action on that resource.
o RS has a valid access token for the sender of the request, but
this does not allow the requested action on the requested
resource.
Note: These conditions ensure that RS can handle requests
autonomously once access was granted and a secure channel has been
established between C and RS. The authz-info endpoint MUST NOT be
protected as specified above, in order to allow clients to upload
access tokens to RS (cf. Section 5.8.1).
Unauthorized Resource Request messages MUST be denied with a client
error response. In this response, the Resource Server SHOULD provide
proper AS Request Creation Hints to enable the Client to request an
access token from RS's AS as described in Section 5.1.2.
The handling of all client requests (including unauthorized ones) by
the RS is described in Section 5.8.2.
5.1.2. AS Request Creation Hints
The AS Request Creation Hints message is sent by RS as a response to
an Unauthorized Resource Request message (see Section 5.1.1) to help
the sender of the Unauthorized Resource Request message in acquiring
a valid access token. The AS Request Creation Hints message is CBOR
map, with a MANDATORY element "AS" specifying an absolute URI (see
Section 4.3 of [RFC3986]) that identifies the AS in charge of RS.
The message can also contain the following OPTIONAL parameters:
o A "audience" element containing a suggested audience that the
client should request towards the AS.
o A "kid" element containing the key identifier of a key used in an
existing security association between the client and the RS. The
RS expects the client to request an access token bound to this
key, in order to avoid having to re-establish the security
association.
o A "cnonce" element containing a client-nonce. See
Section 5.1.2.1.
o A "scope" element containing the suggested scope that the client
should request towards the AS.
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Figure 2 summarizes the parameters that may be part of the AS Request
Creation Hints.
/-----------+----------+---------------------\
| Name | CBOR Key | Value Type |
|-----------+----------+---------------------|
| AS | 1 | text string |
| kid | 2 | byte string |
| audience | 5 | text string |
| scope | 9 | text or byte string |
| cnonce | 39 | byte string |
\-----------+----------+---------------------/
Figure 2: AS Request Creation Hints
Note that the schema part of the AS parameter may need to be adapted
to the security protocol that is used between the client and the AS.
Thus the example AS value "coap://as.example.com/token" might need to
be transformed to "coaps://as.example.com/token". It is assumed that
the client can determine the correct schema part on its own depending
on the way it communicates with the AS.
Figure 3 shows an example for an AS Request Creation Hints message
payload using CBOR [RFC7049] diagnostic notation, using the parameter
names instead of the CBOR keys for better human readability.
4.01 Unauthorized
Content-Format: application/ace+cbor
Payload :
{
"AS" : "coaps://as.example.com/token",
"audience" : "coaps://rs.example.com"
"scope" : "rTempC",
"cnonce" : h'e0a156bb3f'
}
Figure 3: AS Request Creation Hints payload example
In this example, the attribute AS points the receiver of this message
to the URI "coaps://as.example.com/token" to request access
permissions. The originator of the AS Request Creation Hints payload
(i.e., RS) uses a local clock that is loosely synchronized with a
time scale common between RS and AS (e.g., wall clock time).
Therefore, it has included a parameter "nonce" (see Section 5.1.2.1).
Figure 4 illustrates the mandatory to use binary encoding of the
message payload shown in Figure 3.
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a4 # map(4)
01 # unsigned(1) (=AS)
78 1c # text(28)
636f6170733a2f2f61732e657861
6d706c652e636f6d2f746f6b656e # "coaps://as.example.com/token"
05 # unsigned(5) (=audience)
76 # text(22)
636f6170733a2f2f72732e657861
6d706c652e636f6d # "coaps://rs.example.com"
09 # unsigned(9) (=scope)
66 # text(6)
7254656d7043 # "rTempC"
18 27 # unsigned(39) (=cnonce)
45 # bytes(5)
e0a156bb3f # "\xE0\xA1V\xBB?"
Figure 4: AS Request Creation Hints example encoded in CBOR
5.1.2.1. The Client-Nonce Parameter
If the RS does not synchronize its clock with the AS, it could be
tricked into accepting old access tokens, that are either expired or
have been compromised. In order to ensure some level of token
freshness in that case, the RS can use the "cnonce" (client-nonce)
parameter. The processing requirements for this parameter are as
follows:
o A RS sending a "cnonce" parameter in an an AS Request Creation
Hints message MUST store information to validate that a given
cnonce is fresh. How this is implemented internally is out of
scope for this specification. Expiration of client-nonces should
be based roughly on the time it would take a client to obtain an
access token after receiving the AS Request Creation Hints
message.
o A client receiving a "cnonce" parameter in an AS Request Creation
Hints message MUST include this in the parameters when requesting
an access token at the AS, using the "cnonce" parameter from
Section 5.6.4.4.
o If an AS grants an access token request containing a "cnonce"
parameter, it MUST include this value in the access token, using
the "cnonce" claim specified in Section 5.8.
o A RS that is using the client-nonce mechanism and that receives an
access token MUST verify that this token contains a cnonce claim,
with a client-nonce value that is fresh according to the
information stored at the first step above. If the cnonce claim
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is not present or if the cnonce claim value is not fresh, it MUST
discard the access token. If this was an interaction with the
authz-info endpoint the RS MUST also respond with an error message
using a response code equivalent to the CoAP code 4.01
(Unauthorized).
5.2. Authorization Grants
To request an access token, the client obtains authorization from the
resource owner or uses its client credentials as grant. The
authorization is expressed in the form of an authorization grant.
The OAuth framework [RFC6749] defines four grant types. The grant
types can be split up into two groups, those granted on behalf of the
resource owner (password, authorization code, implicit) and those for
the client (client credentials). Further grant types have been added
later, such as [RFC7521] defining an assertion-based authorization
grant.
The grant type is selected depending on the use case. In cases where
the client acts on behalf of the resource owner, authorization code
grant is recommended. If the client acts on behalf of the resource
owner, but does not have any display or very limited interaction
possibilities it is recommended to use the device code grant defined
in [I-D.ietf-oauth-device-flow]. In cases where the client does not
acts autonomously the client credentials grant is recommended.
For details on the different grant types, see section 1.3 of
[RFC6749]. The OAuth 2.0 framework provides an extension mechanism
for defining additional grant types so profiles of this framework MAY
define additional grant types, if needed.
5.3. Client Credentials
Authentication of the client is mandatory independent of the grant
type when requesting the access token from the token endpoint. In
the case of client credentials grant type, the authentication and
grant coincide.
Client registration and provisioning of client credentials to the
client is out of scope for this specification.
The OAuth framework defines one client credential type in section
2.3.1 of [RFC6749]: client id and client secret.
[I-D.erdtman-ace-rpcc] adds raw-public-key and pre-shared-key to the
client credentials types. Profiles of this framework MAY extend with
additional client credentials client certificates.
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5.4. AS Authentication
Client credential does not, by default, authenticate the AS that the
client connects to. In classic OAuth, the AS is authenticated with a
TLS server certificate.
Profiles of this framework MUST specify how clients authenticate the
AS and how communication security is implemented, otherwise server
side TLS certificates, as defined by OAuth 2.0, are required.
5.5. The Authorization Endpoint
The authorization endpoint is used to interact with the resource
owner and obtain an authorization grant in certain grant flows. The
primary use case for this framework is machine-to-machine
interactions, not involving the resource owner in the authorization
flow, therefore this endpoint is out of scope here. Future profiles
may define constrained adaptation mechanisms for this endpoint as
well. Non-constrained clients interacting with constrained resource
servers can use the specifications in section 3.1 of [RFC6749] and of
section 4.2 of [RFC6819].
5.6. The Token Endpoint
In standard OAuth 2.0, the AS provides the token endpoint for
submitting access token requests. This framework extends the
functionality of the token endpoint, giving the AS the possibility to
help the client and RS to establish shared keys or to exchange their
public keys. Furthermore, this framework defines encodings using
CBOR, as a substitute for JSON.
The endpoint may, however, be exposed over HTTPS as in classical
OAuth or even other transports. A profile MUST define the details of
the mapping between the fields described below, and these transports.
If HTTPS is used, JSON or CBOR payloads may be supported. If JSON
payloads are used, the semantics of Section 4 of the OAuth 2.0
specification MUST be followed (with additions as described below).
If CBOR payload is supported, the semantics described below MUST be
followed.
For the AS to be able to issue a token, the client MUST be
authenticated and present a valid grant for the scopes requested.
Profiles of this framework MUST specify how the AS authenticates the
client and how the communication between client and AS is protected.
The default name of this endpoint in an url-path is '/token', however
implementations are not required to use this name and can define
their own instead.
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The figures of this section use CBOR diagnostic notation without the
integer abbreviations for the parameters or their values for
illustrative purposes. Note that implementations MUST use the
integer abbreviations and the binary CBOR encoding, if the CBOR
encoding is used.
5.6.1. Client-to-AS Request
The client sends a POST request to the token endpoint at the AS. The
profile MUST specify how the communication is protected. The content
of the request consists of the parameters specified in the relevant
subsection of section 4 of the OAuth 2.0 specification [RFC6749],
depending on the grant type with the following exceptions and
additions:
o The parameter "grant_type" is OPTIONAL in the context of this
framework (as opposed to REQUIRED in RFC6749). If that parameter
is missing, the default value "client_credentials" is implied.
o The "audience" parameter from [I-D.ietf-oauth-token-exchange] is
OPTIONAL to request an access token bound to a specific audience.
o The "cnonce" parameter defined in Section 5.6.4.4 is REQUIRED if
the RS provided a client-nonce in the "AS Request Creation Hints"
message Section 5.1.2
o The "scope" parameter MAY be encoded as a byte string instead of
the string encoding specified in section 3.3 of [RFC6749], in
order allow compact encoding of complex scopes.
o The client can send an empty (null value) "profile" parameter to
indicate that it wants the AS to include the "profile" parameter
in the response. See Section 5.6.4.3.
o A client MUST be able to use the parameters from
[I-D.ietf-ace-oauth-params] in an access token request to the
token endpoint and the AS MUST be able to process these additional
parameters.
If CBOR is used then these parameters MUST be encoded as a CBOR map.
When HTTP is used as a transport then the client makes a request to
the token endpoint by sending the parameters using the "application/
x-www-form-urlencoded" format with a character encoding of UTF-8 in
the HTTP request entity-body, as defined in section 3.2 of [RFC6749].
The following examples illustrate different types of requests for
proof-of-possession tokens.
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Figure 5 shows a request for a token with a symmetric proof-of-
possession key. The content is displayed in CBOR diagnostic
notation, without abbreviations for better readability.
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"client_id" : "myclient",
"audience" : "tempSensor4711"
}
Figure 5: Example request for an access token bound to a symmetric
key.
Figure 6 shows a request for a token with an asymmetric proof-of-
possession key. Note that in this example OSCORE
[I-D.ietf-core-object-security] is used to provide object-security,
therefore the Content-Format is "application/oscore" wrapping the
"application/ace+cbor" type content. Also note that in this example
the audience is implicitly known by both client and AS. Furthermore
note that this example uses the "req_cnf" parameter from
[I-D.ietf-ace-oauth-params].
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Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
OSCORE: 0x19, 0x05, 0x05, 0x44, 0x61, 0x6c, 0x65, 0x6b
Content-Format: "application/oscore"
Payload:
0x44025d1 ... (full payload omitted for brevity) ... 68b3825e
Decrypted payload:
{
"client_id" : "myclient",
"req_cnf" : {
"COSE_Key" : {
"kty" : "EC",
"kid" : h'11',
"crv" : "P-256",
"x" : b64'usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8',
"y" : b64'IBOL+C3BttVivg+lSreASjpkttcsz+1rb7btKLv8EX4'
}
}
}
Figure 6: Example token request bound to an asymmetric key.
Figure 7 shows a request for a token where a previously communicated
proof-of-possession key is only referenced. Note that the client
performs a password based authentication in this example by
submitting its client_secret (see Section 2.3.1 of [RFC6749]). Note
that this example uses the "req_cnf" parameter from
[I-D.ietf-ace-oauth-params].
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "token"
Content-Format: "application/ace+cbor"
Payload:
{
"client_id" : "myclient",
"client_secret" : "mysecret234",
"audience" : "valve424",
"scope" : "read",
"req_cnf" : {
"kid" : b64'6kg0dXJM13U'
}
}
Figure 7: Example request for an access token bound to a key
reference.
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Refresh tokens are typically not stored as securely as proof-of-
possession keys in requesting clients. Proof-of-possession based
refresh token requests MUST NOT request different proof-of-possession
keys or different audiences in token requests. Refresh token
requests can only use to request access tokens bound to the same
proof-of-possession key and the same audience as access tokens issued
in the initial token request.
5.6.2. AS-to-Client Response
If the access token request has been successfully verified by the AS
and the client is authorized to obtain an access token corresponding
to its access token request, the AS sends a response with the
response code equivalent to the CoAP response code 2.01 (Created).
If client request was invalid, or not authorized, the AS returns an
error response as described in Section 5.6.3.
Note that the AS decides which token type and profile to use when
issuing a successful response. It is assumed that the AS has prior
knowledge of the capabilities of the client and the RS (see
Appendix D). This prior knowledge may, for example, be set by the
use of a dynamic client registration protocol exchange [RFC7591].
The content of the successful reply is the Access Information. When
using CBOR payloads, the content MUST be encoded as CBOR map,
containing parameters as specified in Section 5.1 of [RFC6749], with
the following additions and changes:
profile:
OPTIONAL unless the request included an empty profile parameter in
which case it is MANDATORY. This indicates the profile that the
client MUST use towards the RS. See Section 5.6.4.3 for the
formatting of this parameter. If this parameter is absent, the AS
assumes that the client implicitly knows which profile to use
towards the RS.
token_type:
This parameter is OPTIONAL, as opposed to 'required' in [RFC6749].
By default implementations of this framework SHOULD assume that
the token_type is "pop". If a specific use case requires another
token_type (e.g., "Bearer") to be used then this parameter is
REQUIRED.
Furthermore [I-D.ietf-ace-oauth-params] defines additional parameters
that the AS MUST be able to use when responding to a request to the
token endpoint.
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Figure 8 summarizes the parameters that may be part of the Access
Information. This does not include the additional parameters
specified in [I-D.ietf-ace-oauth-params].
/-------------------+-------------------------------\
| Parameter name | Specified in |
|-------------------+-------------------------------|
| access_token | RFC 6749 |
| token_type | RFC 6749 |
| expires_in | RFC 6749 |
| refresh_token | RFC 6749 |
| scope | RFC 6749 |
| state | RFC 6749 |
| error | RFC 6749 |
| error_description | RFC 6749 |
| error_uri | RFC 6749 |
| profile | [this document] |
\-------------------+-------------------------------/
Figure 8: Access Information parameters
Figure 9 shows a response containing a token and a "cnf" parameter
with a symmetric proof-of-possession key, which is defined in
[I-D.ietf-ace-oauth-params].
Header: Created (Code=2.01)
Content-Format: "application/ace+cbor"
Payload:
{
"access_token" : b64'SlAV32hkKG ...
(remainder of CWT omitted for brevity;
CWT contains COSE_Key in the "cnf" claim)',
"profile" : "coap_dtls",
"expires_in" : "3600",
"cnf" : {
"COSE_Key" : {
"kty" : "Symmetric",
"kid" : b64'39Gqlw',
"k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
}
}
}
Figure 9: Example AS response with an access token bound to a
symmetric key.
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5.6.3. Error Response
The error responses for CoAP-based interactions with the AS are
equivalent to the ones for HTTP-based interactions as defined in
Section 5.2 of [RFC6749], with the following differences:
o When using CBOR the raw payload before being processed by the
communication security protocol MUST be encoded as a CBOR map.
o A response code equivalent to the CoAP code 4.00 (Bad Request)
MUST be used for all error responses, except for invalid_client
where a response code equivalent to the CoAP code 4.01
(Unauthorized) MAY be used under the same conditions as specified
in Section 5.2 of [RFC6749].
o The content type (for CoAP-based interactions) or media type (for
HTTP-based interactions) "application/ace+cbor" MUST be used for
the error response.
o The parameters "error", "error_description" and "error_uri" MUST
be abbreviated using the codes specified in Figure 12, when a CBOR
encoding is used.
o The error code (i.e., value of the "error" parameter) MUST be
abbreviated as specified in Figure 10, when a CBOR encoding is
used.
/------------------------+-------------\
| Name | CBOR Values |
|------------------------+-------------|
| invalid_request | 1 |
| invalid_client | 2 |
| invalid_grant | 3 |
| unauthorized_client | 4 |
| unsupported_grant_type | 5 |
| invalid_scope | 6 |
| unsupported_pop_key | 7 |
| incompatible_profiles | 8 |
\------------------------+-------------/
Figure 10: CBOR abbreviations for common error codes
In addition to the error responses defined in OAuth 2.0, the
following behavior MUST be implemented by the AS:
o If the client submits an asymmetric key in the token request that
the RS cannot process, the AS MUST reject that request with a
response code equivalent to the CoAP code 4.00 (Bad Request)
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including the error code "unsupported_pop_key" defined in
Figure 10.
o If the client and the RS it has requested an access token for do
not share a common profile, the AS MUST reject that request with a
response code equivalent to the CoAP code 4.00 (Bad Request)
including the error code "incompatible_profiles" defined in
Figure 10.
5.6.4. Request and Response Parameters
This section provides more detail about the new parameters that can
be used in access token requests and responses, as well as
abbreviations for more compact encoding of existing parameters and
common parameter values.
5.6.4.1. Grant Type
The abbreviations in Figure 11 MUST be used in CBOR encodings instead
of the string values defined in [RFC6749], if CBOR payloads are used.
/--------------------+------------+------------------------\
| Name | CBOR Value | Original Specification |
|--------------------+------------+------------------------|
| password | 0 | RFC6749 |
| authorization_code | 1 | RFC6749 |
| client_credentials | 2 | RFC6749 |
| refresh_token | 3 | RFC6749 |
\--------------------+------------+------------------------/
Figure 11: CBOR abbreviations for common grant types
5.6.4.2. Token Type
The "token_type" parameter, defined in section 5.1 of [RFC6749],
allows the AS to indicate to the client which type of access token it
is receiving (e.g., a bearer token).
This document registers the new value "pop" for the OAuth Access
Token Types registry, specifying a proof-of-possession token. How
the proof-of-possession by the client to the RS is performed MUST be
specified by the profiles.
The values in the "token_type" parameter MUST be CBOR text strings,
if a CBOR encoding is used.
In this framework the "pop" value for the "token_type" parameter is
the default. The AS may, however, provide a different value.
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5.6.4.3. Profile
Profiles of this framework MUST define the communication protocol and
the communication security protocol between the client and the RS.
The security protocol MUST provide encryption, integrity and replay
protection. It MUST also provide a binding between requests and
responses. Furthermore profiles MUST define proof-of-possession
methods, if they support proof-of-possession tokens.
A profile MUST specify an identifier that MUST be used to uniquely
identify itself in the "profile" parameter. The textual
representation of the profile identifier is just intended for human
readability and MUST NOT be used in parameters and claims.
Profiles MAY define additional parameters for both the token request
and the Access Information in the access token response in order to
support negotiation or signaling of profile specific parameters.
Clients that want the AS to provide them with the "profile" parameter
in the access token response can indicate that by sending a profile
parameter with a null value in the access token request.
5.6.4.4. Client-Nonce
This parameter MUST be sent from the client to the AS, if it
previously received a "cnonce" parameter in the AS Request Creation
Hints Section 5.1.2. The parameter is encoded as a byte string and
copies the value from the cnonce parameter in the AS Request Creation
Hints.
5.6.5. Mapping Parameters to CBOR
If CBOR encoding is used, all OAuth parameters in access token
requests and responses MUST be mapped to CBOR types as specified in
Figure 12, using the given integer abbreviation for the map keys.
Note that we have aligned the abbreviations corresponding to claims
with the abbreviations defined in [RFC8392].
Note also that abbreviations from -24 to 23 have a 1 byte encoding
size in CBOR. We have thus chosen to assign abbreviations in that
range to parameters we expect to be used most frequently in
constrained scenarios.
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/-------------------+----------+---------------------\
| Name | CBOR Key | Value Type |
|-------------------+----------+---------------------|
| access_token | 1 | byte string |
| expires_in | 2 | unsigned integer |
| audience | 5 | text string |
| scope | 9 | text or byte string |
| client_id | 24 | text string |
| client_secret | 25 | byte string |
| response_type | 26 | text string |
| redirect_uri | 27 | text string |
| state | 28 | text string |
| code | 29 | byte string |
| error | 30 | unsigned integer |
| error_description | 31 | text string |
| error_uri | 32 | text string |
| grant_type | 33 | unsigned integer |
| token_type | 34 | unsigned integer |
| username | 35 | text string |
| password | 36 | text string |
| refresh_token | 37 | byte string |
| profile | 38 | unsigned integer |
| cnonce | 39 | byte string |
\-------------------+----------+---------------------/
Figure 12: CBOR mappings used in token requests
5.7. The Introspection Endpoint
Token introspection [RFC7662] can be OPTIONALLY provided by the AS,
and is then used by the RS and potentially the client to query the AS
for metadata about a given token, e.g., validity or scope. Analogous
to the protocol defined in [RFC7662] for HTTP and JSON, this section
defines adaptations to more constrained environments using CBOR and
leaving the choice of the application protocol to the profile.
Communication between the requesting entity and the introspection
endpoint at the AS MUST be integrity protected and encrypted. The
communication security protocol MUST also provide a binding between
requests and responses. Furthermore the two interacting parties MUST
perform mutual authentication. Finally the AS SHOULD verify that the
requesting entity has the right to access introspection information
about the provided token. Profiles of this framework that support
introspection MUST specify how authentication and communication
security between the requesting entity and the AS is implemented.
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The default name of this endpoint in an url-path is '/introspect',
however implementations are not required to use this name and can
define their own instead.
The figures of this section uses CBOR diagnostic notation without the
integer abbreviations for the parameters or their values for better
readability.
Note that supporting introspection is OPTIONAL for implementations of
this framework.
5.7.1. Introspection Request
The requesting entity sends a POST request to the introspection
endpoint at the AS, the profile MUST specify how the communication is
protected. If CBOR is used, the payload MUST be encoded as a CBOR
map with a "token" entry containing either the access token or a
reference to the token (e.g., the cti). Further optional parameters
representing additional context that is known by the requesting
entity to aid the AS in its response MAY be included.
For CoAP-based interaction, all messages MUST use the content type
"application/ace+cbor", while for HTTP-based interactions the
equivalent media type "application/ace+cbor" MUST be used.
The same parameters are required and optional as in Section 2.1 of
[RFC7662].
For example, Figure 13 shows a RS calling the token introspection
endpoint at the AS to query about an OAuth 2.0 proof-of-possession
token. Note that object security based on OSCORE
[I-D.ietf-core-object-security] is assumed in this example, therefore
the Content-Format is "application/oscore". Figure 14 shows the
decoded payload.
Header: POST (Code=0.02)
Uri-Host: "as.example.com"
Uri-Path: "introspect"
OSCORE: 0x09, 0x05, 0x25
Content-Format: "application/oscore"
Payload:
... COSE content ...
Figure 13: Example introspection request.
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{
"token" : b64'7gj0dXJQ43U',
"token_type_hint" : "pop"
}
Figure 14: Decoded token.
5.7.2. Introspection Response
If the introspection request is authorized and successfully
processed, the AS sends a response with the response code equivalent
to the CoAP code 2.01 (Created). If the introspection request was
invalid, not authorized or couldn't be processed the AS returns an
error response as described in Section 5.7.3.
In a successful response, the AS encodes the response parameters in a
map including with the same required and optional parameters as in
Section 2.2 of [RFC7662] with the following addition:
profile OPTIONAL. This indicates the profile that the RS MUST use
with the client. See Section 5.6.4.3 for more details on the
formatting of this parameter.
cnonce OPTIONAL. A client-nonce previously provided to the AS by
the RS via the client. See Section 5.6.4.4.
exi OPTIONAL. The "expires-in" claim associated to this access
token. See Section 5.8.3.
Furthermore [I-D.ietf-ace-oauth-params] defines more parameters that
the AS MUST be able to use when responding to a request to the
introspection endpoint.
For example, Figure 15 shows an AS response to the introspection
request in Figure 13. Note that this example contains the "cnf"
parameter defined in [I-D.ietf-ace-oauth-params].
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Header: Created (Code=2.01)
Content-Format: "application/ace+cbor"
Payload:
{
"active" : true,
"scope" : "read",
"profile" : "coap_dtls",
"cnf" : {
"COSE_Key" : {
"kty" : "Symmetric",
"kid" : b64'39Gqlw',
"k" : b64'hJtXhkV8FJG+Onbc6mxCcQh'
}
}
}
Figure 15: Example introspection response.
5.7.3. Error Response
The error responses for CoAP-based interactions with the AS are
equivalent to the ones for HTTP-based interactions as defined in
Section 2.3 of [RFC7662], with the following differences:
o If content is sent and CBOR is used the payload MUST be encoded as
a CBOR map and the Content-Format "application/ace+cbor" MUST be
used.
o If the credentials used by the requesting entity (usually the RS)
are invalid the AS MUST respond with the response code equivalent
to the CoAP code 4.01 (Unauthorized) and use the required and
optional parameters from Section 5.2 in [RFC6749].
o If the requesting entity does not have the right to perform this
introspection request, the AS MUST respond with a response code
equivalent to the CoAP code 4.03 (Forbidden). In this case no
payload is returned.
o The parameters "error", "error_description" and "error_uri" MUST
be abbreviated using the codes specified in Figure 12.
o The error codes MUST be abbreviated using the codes specified in
Figure 10.
Note that a properly formed and authorized query for an inactive or
otherwise invalid token does not warrant an error response by this
specification. In these cases, the authorization server MUST instead
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respond with an introspection response with the "active" field set to
"false".
5.7.4. Mapping Introspection parameters to CBOR
If CBOR is used, the introspection request and response parameters
MUST be mapped to CBOR types as specified in Figure 16, using the
given integer abbreviation for the map key.
Note that we have aligned abbreviations that correspond to a claim
with the abbreviations defined in [RFC8392] and the abbreviations of
parameters with the same name from Section 5.6.5.
/-------------------+----------+-------------------------\
| Parameter name | CBOR Key | Value Type |
|-------------------+----------+-------------------------|
| iss | 1 | text string |
| sub | 2 | text string |
| aud | 3 | text string |
| exp | 4 | integer or |
| | | floating-point number |
| nbf | 5 | integer or |
| | | floating-point number |
| iat | 6 | integer or |
| | | floating-point number |
| cti | 7 | byte string |
| scope | 9 | text or byte string |
| active | 10 | True or False |
| token | 11 | byte string |
| client_id | 24 | text string |
| error | 30 | unsigned integer |
| error_description | 31 | text string |
| error_uri | 32 | text string |
| token_type_hint | 33 | text string |
| token_type | 34 | text string |
| username | 35 | text string |
| profile | 38 | unsigned integer |
| cnonce | 39 | byte string |
| exi | 40 | unsigned integer |
\-------------------+----------+-------------------------/
Figure 16: CBOR Mappings to Token Introspection Parameters.
5.8. The Access Token
This framework RECOMMENDS the use of CBOR web token (CWT) as
specified in [RFC8392].
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In order to facilitate offline processing of access tokens, this
document uses the "cnf" claim from
[I-D.ietf-ace-cwt-proof-of-possession] and specifies the "scope"
claim for JWT- and CWT-encoded tokens.
The "scope" claim explicitly encodes the scope of a given access
token. This claim follows the same encoding rules as defined in
Section 3.3 of [RFC6749], but in addition implementers MAY use byte
strings as scope values, to achieve compact encoding of large scope
elements. The meaning of a specific scope value is application
specific and expected to be known to the RS running that application.
If the AS needs to convey a hint to the RS about which profile it
should use to communicate with the client, the AS MAY include a
"profile" claim in the access token, with the same syntax and
semantics as defined in Section 5.6.4.3.
If the client submitted a client-nonce parameter in the access token
request Section 5.6.4.4, the AS MUST include the value of this
parameter in the "cnonce" claim specified here. The "cnonce" claim
uses binary encoding.
5.8.1. The Authorization Information Endpoint
The access token, containing authorization information and
information about the key used by the client, needs to be transported
to the RS so that the RS can authenticate and authorize the client
request.
This section defines a method for transporting the access token to
the RS using a RESTful protocol such as CoAP. Profiles of this
framework MAY define other methods for token transport.
The method consists of an authz-info endpoint, implemented by the RS.
A client using this method MUST make a POST request to the authz-info
endpoint at the RS with the access token in the payload. The RS
receiving the token MUST verify the validity of the token. If the
token is valid, the RS MUST respond to the POST request with 2.01
(Created). Section Section 5.8.1.1 outlines how an RS MUST proceed
to verify the validity of an access token.
The RS MUST be prepared to store at least one access token for future
use. This is a difference to how access tokens are handled in OAuth
2.0, where the access token is typically sent along with each
request, and therefore not stored at the RS.
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This specification RECOMMENDS that an RS stores only one token per
proof-of-possession key, meaning that an additional token linked to
the same key will overwrite any existing token at the RS.
If the payload sent to the authz-info endpoint does not parse to a
token, the RS MUST respond with a response code equivalent to the
CoAP code 4.00 (Bad Request).
The RS MAY make an introspection request to validate the token before
responding to the POST request to the authz-info endpoint.
Profiles MUST specify whether the authz-info endpoint is protected,
including whether error responses from this endpoint are protected.
Note that since the token contains information that allow the client
and the RS to establish a security context in the first place, mutual
authentication may not be possible at this point.
The default name of this endpoint in an url-path is '/authz-info',
however implementations are not required to use this name and can
define their own instead.
A RS MAY use introspection on a token received through the authz-info
endpoint, e.g. if the token is an opaque reference. Some transport
protocols may provide a way to indicate that the RS is busy and the
client should retry after an interval; this type of status update
would be appropriate while the RS is waiting for an introspection
response.
5.8.1.1. Verifying an Access Token
When an RS receives an access token, it MUST verify it before storing
it. The details of token verification depends on various aspects,
including the token encoding, the type of token, the security
protection applied to the token, and the claims. The token encoding
matters since the security wrapper differs between the token
encodings. For example, a CWT token uses COSE while a JWT token uses
JOSE. The type of token also has an influence on the verification
procedure since tokens may be self-contained whereby token
verification may happen locally at the RS while a token-by-reference
requires further interaction with the authorization server, for
example using token introspection, to obtain the claims associated
with the token reference. Self-contained token MUST, at a minimum,
be integrity protected but they MAY also be encrypted.
For self-contained tokens the RS MUST process the security protection
of the token first, as specified by the respective token format. For
CWT the description can be found in [RFC8392] and for JWT the
relevant specification is [RFC7519]. This MUST include a
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verification that security protection (and thus the token) was
generated by an AS that has the right to issue access tokens for this
RS.
In case the token is communicated by reference the RS needs to obtain
the claims first. When the RS uses token introspection the relevant
specification is [RFC7662] with CoAP transport specified in
Section 5.7.
Errors may happen during this initial processing stage:
o If token or claim verification fails, the RS MUST discard the
token and, if this was an interaction with authz-info, return an
error message with a response code equivalent to the CoAP code
4.01 (Unauthorized).
o If the claims cannot be obtained the RS MUST discard the token
and, in case of an interaction via the authz-info endpoint, return
an error message with a response code equivalent to the CoAP code
4.00 (Bad Request).
Next, the RS MUST verify claims, if present, contained in the access
token. Errors are returned when claim checks fail, in the order of
priority of this list:
iss The issuer claim must identify an AS that has the authority to
issue access tokens for the receiving RS. If that is not the case
the RS MUST discard the token. If this was an interaction with
authz-info, the RS MUST also respond with a response code
equivalent to the CoAP code 4.01 (Unauthorized).
exp The expiration date must be in the future. If that is not the
case the RS MUST discard the token. If this was an interaction
with authz-info the RS MUST also respond with a response code
equivalent to the CoAP code 4.01 (Unauthorized). Note that the RS
has to terminate access rights to the protected resources at the
time when the tokens expire.
aud The audience claim must refer to an audience that the RS
identifies with. If that is not the case the RS MUST discard the
token. If this was an interaction with authz-info, the RS MUST
also respond with a response code equivalent to the CoAP code 4.03
(Forbidden).
scope The RS must recognize value of the scope claim. If that is
not the case the RS MUST discard the token. If this was an
interaction with authz-info, the RS MUST also respond with a
response code equivalent to the CoAP code 4.00 (Bad Request). The
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RS MAY provide additional information in the error response, to
clarify what went wrong.
If the access token contains any other claims that the RS cannot
process the RS MUST discard the token. If this was an interaction
with authz-info, the RS MUST also respond with a response code
equivalent to the CoAP code 4.00 (Bad Request). The RS MAY provide
additional detail in the error response to clarify which claim
couldn't be processed.
Note that the Subject (sub) claim cannot always be verified when the
token is submitted to the RS since the client may not have
authenticated yet. Also note that a counter for the expires_in (exi)
claim MUST be initialized when the RS first verifies this token.
Also note that profiles of this framework may define access token
transport mechanisms that do not allow for error responses.
Therefore the error messages specified here only apply if the token
was POSTed to the authz-info endpoint.
When sending error responses, the RS MAY use the error codes from
Section 3.1 of [RFC6750], to provide additional details to the
client.
5.8.1.2. Protecting the Authorization Information Endpoint
As this framework can be used in RESTful environments, it is
important to make sure that attackers cannot perform unauthorized
requests on the auth-info endpoints, other than submitting access
tokens.
Specifically it SHOULD NOT be possible to perform GET, DELETE or PUT
on the authz-info endpoint and on it's children (if any).
The POST method SHOULD NOT be allowed on children of the authz-info
endpoint.
The RS SHOULD implement rate limiting measures to mitigate attacks
aiming to overload the processing capacity of the RS by repeatedly
submitting tokens. For CoAP-based communication the RS could use the
mechanisms from [RFC8516] to indicate that it is overloaded.
5.8.2. Client Requests to the RS
Before sending a request to a RS, the client MUST verify that the
keys used to protect this communication are still valid. See
Section 5.8.4 for details on how the client determines the validity
of the keys used.
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If an RS receives a request from a client, and the target resource
requires authorization, the RS MUST first verify that it has an
access token that authorizes this request, and that the client has
performed the proof-of-possession for that token.
The response code MUST be 4.01 (Unauthorized) in case the client has
not performed the proof-of-possession, or if RS has no valid access
token for the client. If RS has an access token for the client but
not for the resource that was requested, RS MUST reject the request
with a 4.03 (Forbidden). If RS has an access token for the client
but it does not cover the action that was requested on the resource,
RS MUST reject the request with a 4.05 (Method Not Allowed).
Note: The use of the response codes 4.03 and 4.05 is intended to
prevent infinite loops where a dumb Client optimistically tries to
access a requested resource with any access token received from AS.
As malicious clients could pretend to be C to determine C's
privileges, these detailed response codes must be used only when a
certain level of security is already available which can be achieved
only when the Client is authenticated.
Note: The RS MAY use introspection for timely validation of an access
token, at the time when a request is presented.
Note: Matching the claims of the access token (e.g., scope) to a
specific request is application specific.
If the request matches a valid token and the client has performed the
proof-of-possession for that token, the RS continues to process the
request as specified by the underlying application.
5.8.3. Token Expiration
Depending on the capabilities of the RS, there are various ways in
which it can verify the expiration of a received access token. Here
follows a list of the possibilities including what functionality they
require of the RS.
o The token is a CWT and includes an "exp" claim and possibly the
"nbf" claim. The RS verifies these by comparing them to values
from its internal clock as defined in [RFC7519]. In this case the
RS's internal clock must reflect the current date and time, or at
least be synchronized with the AS's clock. How this clock
synchronization would be performed is out of scope for this
specification.
o The RS verifies the validity of the token by performing an
introspection request as specified in Section 5.7. This requires
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the RS to have a reliable network connection to the AS and to be
able to handle two secure sessions in parallel (C to RS and AS to
RS).
o In order to support token expiration for devices that have no
reliable way of synchronizing their internal clocks, this
specification defines the following approach: The claim "exi"
("expires in") can be used, to provide the RS with the lifetime of
the token in seconds from the time the RS first receives the
token. This approach is of course vulnerable to malicious clients
holding back tokens they do not want to expire. Such an attack
can only be prevented if the RS is able to communicate with the AS
in some regular intervals, so that the can AS provide the RS with
a list of expired tokens. The drawback of this mitigation is that
the RS might as well use the communication with the AS to
synchronize its internal clock.
If a token that authorizes a long running request such as a CoAP
Observe [RFC7641] expires, the RS MUST send an error response with
the response code equivalent to the CoAP code 4.01 (Unauthorized) to
the client and then terminate processing the long running request.
5.8.4. Key Expiration
The AS provides the client with key material that the RS uses. This
can either be a common symmetric pop-key, or an asymmetric key used
by the RS to authenticate towards the client. Since there is no
metadata associated to those keys, the client has no way of knowing
if these keys are still valid. This may lead to situations where the
client sends requests containing sensitive information to the RS
using a key that is expired and possibly in the hands of an attacker,
or accepts responses from the RS that are not properly protected and
could possibly have been forged by an attacker.
In order to prevent this, the client must assume that those keys are
only valid as long as the related access token is. Since the access
token is opaque to the client, one of the following methods MUST be
used to inform the client about the validity of an access token:
o The client knows a default validity period for all tokens it is
using. This information could be provisioned to the client when
it is registered at the AS, or published by the AS in a way that
the client can query.
o The AS informs the client about the token validity using the
"expires_in" parameter in the Access Information.
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o The client performs an introspection of the token. Although this
is not explicitly forbidden, how exactly a client does
introspection is not currently specified for OAuth.
A client that is not able to obtain information about the expiration
of a token MUST NOT use this token.
6. Security Considerations
Security considerations applicable to authentication and
authorization in RESTful environments provided in OAuth 2.0 [RFC6749]
apply to this work. Furthermore [RFC6819] provides additional
security considerations for OAuth which apply to IoT deployments as
well. If the introspection endpoint is used, the security
considerations from [RFC7662] also apply.
A large range of threats can be mitigated by protecting the contents
of the access token by using a digital signature or a keyed message
digest (MAC) or an Authenticated Encryption with Associated Data
(AEAD) algorithm. Consequently, the token integrity protection MUST
be applied to prevent the token from being modified, particularly
since it contains a reference to the symmetric key or the asymmetric
key. If the access token contains the symmetric key, this symmetric
key MUST be encrypted by the authorization server so that only the
resource server can decrypt it. Note that using an AEAD algorithm is
preferable over using a MAC unless the message needs to be publicly
readable.
If the token is intended for multiple recipients (i.e. an audience
that is a group), integrity protection of the token with a symmetric
key is not sufficient, since any of the recipients could modify the
token undetected by the other recipients. Therefore a token with a
multi-recipient audience MUST be protected with an asymmetric
signature.
It is important for the authorization server to include the identity
of the intended recipient (the audience), typically a single resource
server (or a list of resource servers), in the token. Using a single
shared secret with multiple resource servers to simplify key
management is NOT RECOMMENDED since the benefit from using the proof-
of-possession concept is significantly reduced.
The authorization server MUST offer confidentiality protection for
any interactions with the client. This step is extremely important
since the client may obtain the proof-of-possession key from the
authorization server for use with a specific access token. Not using
confidentiality protection exposes this secret (and the access token)
to an eavesdropper thereby completely negating proof-of-possession
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security. Profiles MUST specify how confidentiality protection is
provided, and additional protection can be applied by encrypting the
token, for example encryption of CWTs is specified in Section 5.1 of
[RFC8392].
Developers MUST ensure that the ephemeral credentials (i.e., the
private key or the session key) are not leaked to third parties. An
adversary in possession of the ephemeral credentials bound to the
access token will be able to impersonate the client. Be aware that
this is a real risk with many constrained environments, since
adversaries can often easily get physical access to the devices.
This risk can also be mitigated to some extent by making sure that
keys are refreshed more frequently.
If clients are capable of doing so, they should frequently request
fresh access tokens, as this allows the AS to keep the lifetime of
the tokens short. This allows the AS to use shorter proof-of-
possession key sizes, which translate to a performance benefit for
the client and for the resource server. Shorter keys also lead to
shorter messages (particularly with asymmetric keying material).
When authorization servers bind symmetric keys to access tokens, they
SHOULD scope these access tokens to a specific permission.
6.1. Unprotected AS Request Creation Hints
Initially, no secure channel exists to protect the communication
between C and RS. Thus, C cannot determine if the AS Request
Creation Hints contained in an unprotected response from RS to an
unauthorized request (see Section 5.1.2) are authentic. It is
therefore advisable to provide C with a (possibly hard-coded) list of
trustworthy authorization servers. AS Request Creation Hints
referring to a URI not listed there would be ignored.
6.2. Minimal security requirements for communication
This section summarizes the minimal requirements for the
communication security of the different protocol interactions.
C-AS All communication between the client and the Authorization
Server MUST be encrypted, integrity and replay protected.
Furthermore responses from the AS to the client MUST be bound to
the client's request to avoid attacks where the attacker swaps the
intended response for an older one valid for a previous request.
This requires that the client and the Authorization Server have
previously exchanged either a shared secret, or their public keys
in order to negotiate a secure communication. Furthermore the
client MUST be able to determine whether an AS has the authority
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to issue access tokens for a certain RS. This can be done through
pre-configured lists, or through an online lookup mechanism that
in turn also must be secured.
RS-AS The communication between the Resource Server and the
Authorization Server via the introspection endpoint MUST be
encrypted, integrity and replay protected. Furthermore responses
from the AS to the RS MUST be bound to the RS's request. This
requires that the client and the Authorization Server have
previously exchanged either a shared secret, or their public keys
in order to negotiate a secure communication. Furthermore the RS
MUST be able to determine whether an AS has the authority to issue
access tokens itself. This is usually configured out of band, but
could also be performed through an online lookup mechanism
provided that it is also secured in the same way.
C-RS The initial communication between the client and the Resource
Server can not be secured in general, since the RS is not in
possession of on access token for that client, which would carry
the necessary parameters. Certain security mechanisms (e.g. DTLS
with server-side authentication via a certificate or a raw public
key) can be possible and are RECOMMEND if supported by both
parties. After the client has successfully transmitted the access
token to the RS, a secure communication protocol MUST be
established between client and RS for the actual resource request.
This protocol MUST provide encryption, integrity and replay
protection as well as a binding between requests and responses.
This requires that the client learned either the RS's public key
or received a symmetric proof-of-possession key bound to the
access token from the AS. The RS must have learned either the
client's public key or a shared symmetric key from the claims in
the token or an introspection request. Since ACE does not provide
profile negotiation between C and RS, the client MUST have learned
what profile the RS supports (e.g. from the AS or pre-configured)
and initiate the communication accordingly.
6.3. Use of Nonces for Token Freshness
An RS that does not synchronize its clock with the AS may be tricked
into accepting old access tokens that are no longer valid or have
been compromised. In order to prevent this, an RS may use the nonce-
based mechanism defined in Section 5.1.2 to ensure freshness of an
Access Token subsequently presented to this RS.
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6.4. Combining profiles
There may be use cases were different profiles of this framework are
combined. For example, an MQTT-TLS profile is used between the
client and the RS in combination with a CoAP-DTLS profile for
interactions between the client and the AS. Ideally, profiles should
be designed in a way that the security of system should not depend on
the specific security mechanisms used in individual protocol
interactions.
6.5. Unprotected Information
Communication with the authz-info endpoint, as well as the various
error responses defined in this framework all potentially include
sending information over an unprotected channel. These messages may
leak information to an adversary. For example errors responses for
requests to the Authorization Information endpoint can reveal
information about an otherwise opaque access token to an adversary
who has intercepted this token.
As far as error messages are concerned, this framework is written
under the assumption that, in general, the benefits of detailed error
messages outweigh the risk due to information leakage. For
particular use cases, where this assessment does not apply, detailed
error messages can be replaced by more generic ones.
In some scenarios it may be possible to protect the communication
with the authz-info endpoint (e.g. through DTLS with only server-side
authentication). In cases where this is not possible this framework
RECOMMENDS to use encrypted CWTs or opaque references and need to be
subjected to introspection by the RS.
If the initial unauthorized resource request message (see
Section 5.1.1) is used, the client MUST make sure that it is not
sending sensitive content in this request. While GET and DELETE
requests only reveal the target URI of the resource, while POST and
PUT requests would reveal the whole payload of the intended
operation.
6.6. Identifying audiences
The audience claim as defined in [RFC7519] and the equivalent
"audience" parameter from [I-D.ietf-oauth-token-exchange] are
intentionally vague on how to match the audience value to a specific
RS. This is intended to allow application specific semantics to be
used. This section attempts to give some general guidance for the
use of audiences in constrained environments.
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URLs are not a good way of identifying mobile devices that can switch
networks and thus be associated with new URLs. If the audience
represents a single RS, and asymmetric keys are used, the RS can be
uniquely identified by a hash of its public key. If this approach is
used this framework RECOMMENDS to apply the procedure from section 3
of [RFC6920].
If the audience addresses a group of resource servers, the mapping of
group identifier to individual RS has to be provisioned to each RS
before the group-audience is usable. Managing dynamic groups could
be an issue, if the RS is not always reachable when the group
memberships change. Furthermore issuing access tokens bound to
symmetric proof-of-possession keys that apply to a group-audience is
problematic, as an RS that is in possession of the access token can
impersonate the client towards the other RSs that are part of the
group. It is therefore NOT RECOMMENDED to issue access tokens bound
to a group audience and symmetric proof-of possession keys.
Even the client must be able to determine the correct values to put
into the "audience" parameter, in order to obtain a token for the
intended RS. Errors in this process can lead to the client
inadvertently communicating with the wrong RS. The correct values
for "audience" can either be provisioned to the client as part of its
configuration, or provided by the RS as part of the "AS Request
Creation Hints" Section 5.1.2 or dynamically looked up by the client
in some directory. In the latter case the integrity and correctness
of the directory data must be assured.
6.7. Denial of service against or with Introspection
The optional introspection mechanism provided by OAuth and supported
in the ACE framework allows for two types of attacks that need to be
considered by implementers.
First an attacker could perform a denial of service attack against
the introspection endpoint at the AS in order to prevent validation
of access tokens. To mitigate this attack, an RS that is configured
to use introspection MUST NOT allow access based on a token for which
it couldn't reach the introspection endpoint.
Second an attacker could use the fact that an RS performs
introspection to perform a denial of service attack against that RS
by repeatedly sending tokens to its authz-info endpoint that require
an introspection call. RS can mitigate such attacks by implementing
a rate limit on how many introspection requests they perform in a
given time interval and rejecting incoming requests to authz-info for
a certain amount of time, when that rate limit has been reached.
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7. Privacy Considerations
Implementers and users should be aware of the privacy implications of
the different possible deployments of this framework.
The AS is in a very central position and can potentially learn
sensitive information about the clients requesting access tokens. If
the client credentials grant is used, the AS can track what kind of
access the client intends to perform. With other grants this can be
prevented by the Resource Owner. To do so, the resource owner needs
to bind the grants it issues to anonymous, ephemeral credentials that
do not allow the AS to link different grants and thus different
access token requests by the same client.
If access tokens are only integrity protected and not encrypted, they
may reveal information to attackers listening on the wire, or able to
acquire the access tokens in some other way. In the case of CWTs the
token may, e.g., reveal the audience, the scope and the confirmation
method used by the client. The latter may reveal the identity of the
device or application running the client. This may be linkable to
the identity of the person using the client (if there is a person and
not a machine-to-machine interaction).
Clients using asymmetric keys for proof-of-possession should be aware
of the consequences of using the same key pair for proof-of-
possession towards different RSs. A set of colluding RSs or an
attacker able to obtain the access tokens will be able to link the
requests, or even to determine the client's identity.
An unprotected response to an unauthorized request (see
Section 5.1.2) may disclose information about RS and/or its existing
relationship with C. It is advisable to include as little
information as possible in an unencrypted response. Means of
encrypting communication between C and RS already exist, more
detailed information may be included with an error response to
provide C with sufficient information to react on that particular
error.
8. IANA Considerations
8.1. ACE Authorization Server Request Creation Hints
This specification establishes the IANA "ACE Authorization Server
Request Creation Hints" registry. The registry has been created to
use the "Expert Review" registration procedure [RFC8126]. It should
be noted that, in addition to the expert review, some portions of the
registry require a specification, potentially a Standards Track RFC,
be supplied as well.
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The columns of the registry are:
Name The name of the parameter
CBOR Key CBOR map key for the parameter. Different ranges of values
use different registration policies [RFC8126]. Integer values
from -256 to 255 are designated as Standards Action. Integer
values from -65536 to -257 and from 256 to 65535 are designated as
Specification Required. Integer values greater than 65535 are
designated as Expert Review. Integer values less than -65536 are
marked as Private Use.
Value Type The CBOR data types allowable for the values of this
parameter.
Reference This contains a pointer to the public specification of the
grant type abbreviation, if one exists.
This registry will be initially populated by the values in Figure 2.
The Reference column for all of these entries will be this document.
8.2. OAuth Extensions Error Registration
This specification registers the following error values in the OAuth
Extensions Error registry defined in [RFC6749].
o Error name: "unsupported_pop_key"
o Error usage location: token error response
o Related protocol extension: The ACE framework [this document]
o Change Controller: IESG
o Specification document(s): Section 5.6.3 of [this document]
o Error name: "incompatible_profiles"
o Error usage location: token error response
o Related protocol extension: The ACE framework [this document]
o Change Controller: IESG
o Specification document(s): Section 5.6.3 of [this document]
8.3. OAuth Error Code CBOR Mappings Registry
This specification establishes the IANA "OAuth Error Code CBOR
Mappings" registry. The registry has been created to use the "Expert
Review" registration procedure [RFC8126], except for the value range
designated for private use.
The columns of the registry are:
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Name The OAuth Error Code name, refers to the name in Section 5.2.
of [RFC6749], e.g., "invalid_request".
CBOR Value CBOR abbreviation for this error code. Integer values
less than -65536 are marked as "Private Use", all other values use
the registration policy "Expert Review" [RFC8126].
Reference This contains a pointer to the public specification of the
grant type abbreviation, if one exists.
This registry will be initially populated by the values in Figure 10.
The Reference column for all of these entries will be this document.
8.4. OAuth Grant Type CBOR Mappings
This specification establishes the IANA "OAuth Grant Type CBOR
Mappings" registry. The registry has been created to use the "Expert
Review" registration procedure [RFC8126], except for the value range
designated for private use.
The columns of this registry are:
Name The name of the grant type as specified in Section 1.3 of
[RFC6749].
CBOR Value CBOR abbreviation for this grant type. Integer values
less than -65536 are marked as "Private Use", all other values use
the registration policy "Expert Review" [RFC8126].
Reference This contains a pointer to the public specification of the
grant type abbreviation, if one exists.
Original Specification This contains a pointer to the public
specification of the grant type, if one exists.
This registry will be initially populated by the values in Figure 11.
The Reference column for all of these entries will be this document.
8.5. OAuth Access Token Types
This section registers the following new token type in the "OAuth
Access Token Types" registry [IANA.OAuthAccessTokenTypes].
o Type name: "PoP"
o Additional Token Endpoint Response Parameters: "cnf", "rs_cnf" see
section 3.3 of [I-D.ietf-ace-oauth-params].
o HTTP Authentication Scheme(s): N/A
o Change Controller: IETF
o Specification document(s): [this document]
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8.6. OAuth Access Token Type CBOR Mappings
This specification established the IANA "OAuth Access Token Type CBOR
Mappings" registry. The registry has been created to use the "Expert
Review" registration procedure [RFC8126], except for the value range
designated for private use.
The columns of this registry are:
Name The name of token type as registered in the OAuth Access Token
Types registry, e.g., "Bearer".
CBOR Value CBOR abbreviation for this token type. Integer values
less than -65536 are marked as "Private Use", all other values use
the registration policy "Expert Review" [RFC8126].
Reference This contains a pointer to the public specification of the
OAuth token type abbreviation, if one exists.
Original Specification This contains a pointer to the public
specification of the grant type, if one exists.
8.6.1. Initial Registry Contents
o Name: "Bearer"
o Value: 1
o Reference: [this document]
o Original Specification: [RFC6749]
o Name: "pop"
o Value: 2
o Reference: [this document]
o Original Specification: [this document]
8.7. ACE Profile Registry
This specification establishes the IANA "ACE Profile" registry. The
registry has been created to use the "Expert Review" registration
procedure [RFC8126]. It should be noted that, in addition to the
expert review, some portions of the registry require a specification,
potentially a Standards Track RFC, be supplied as well.
The columns of this registry are:
Name The name of the profile, to be used as value of the profile
attribute.
Description Text giving an overview of the profile and the context
it is developed for.
CBOR Value CBOR abbreviation for this profile name. Different
ranges of values use different registration policies [RFC8126].
Integer values from -256 to 255 are designated as Standards
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Action. Integer values from -65536 to -257 and from 256 to 65535
are designated as Specification Required. Integer values greater
than 65535 are designated as "Expert Review". Integer values less
than -65536 are marked as Private Use.
Reference This contains a pointer to the public specification of the
profile abbreviation, if one exists.
This registry will be initially empty and will be populated by the
registrations from the ACE framework profiles.
8.8. OAuth Parameter Registration
This specification registers the following parameter in the "OAuth
Parameters" registry [IANA.OAuthParameters]:
o Name: "profile"
o Parameter Usage Location: token response
o Change Controller: IESG
o Reference: Section 5.6.4.3 of [this document]
8.9. OAuth Parameters CBOR Mappings Registry
This specification establishes the IANA "OAuth Parameters CBOR
Mappings" registry. The registry has been created to use the "Expert
Review" registration procedure [RFC8126], except for the value range
designated for private use.
The columns of this registry are:
Name The OAuth Parameter name, refers to the name in the OAuth
parameter registry, e.g., "client_id".
CBOR Key CBOR map key for this parameter. Integer values less than
-65536 are marked as "Private Use", all other values use the
registration policy "Expert Review" [RFC8126].
Value Type The allowable CBOR data types for values of this
parameter.
Reference This contains a pointer to the public specification of the
parameter abbreviation, if one exists.
This registry will be initially populated by the values in Figure 12.
The Reference column for all of these entries will be this document.
Note that the mappings of parameters corresponding to claim names
intentionally coincide with the CWT claim name mappings from
[RFC8392].
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8.10. OAuth Introspection Response Parameter Registration
This specification registers the following parameter in the OAuth
Token Introspection Response registry
[IANA.TokenIntrospectionResponse].
o Name: "profile"
o Description: The communication and communication security profile
used between client and RS, as defined in ACE profiles.
o Change Controller: IESG
o Reference: Section 5.7.2 of [this document]
8.11. OAuth Token Introspection Response CBOR Mappings Registry
This specification establishes the IANA "OAuth Token Introspection
Response CBOR Mappings" registry. The registry has been created to
use the "Expert Review" registration procedure [RFC8126], except for
the value range designated for private use.
The columns of this registry are:
Name The OAuth Parameter name, refers to the name in the OAuth
parameter registry, e.g., "client_id".
CBOR Key CBOR map key for this parameter. Integer values less than
-65536 are marked as "Private Use", all other values use the
registration policy "Expert Review" [RFC8126].
Value Type The allowable CBOR data types for values of this
parameter.
Reference This contains a pointer to the public specification of the
grant type abbreviation, if one exists.
This registry will be initially populated by the values in Figure 16.
The Reference column for all of these entries will be this document.
Note that the mappings of parameters corresponding to claim names
intentionally coincide with the CWT claim name mappings from
[RFC8392].
8.12. JSON Web Token Claims
This specification registers the following new claims in the JSON Web
Token (JWT) registry of JSON Web Token Claims
[IANA.JsonWebTokenClaims]:
o Claim Name: "scope"
o Claim Description: The scope of an access token as defined in
[RFC6749].
o Change Controller: IESG
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o Reference: Section 5.8 of [this document]
o Claim Name: "profile"
o Claim Description: The profile a token is supposed to be used
with.
o Change Controller: IESG
o Reference: Section 5.8 of [this document]
o Claim Name: "exi"
o Claim Description: "Expires in". Lifetime of the token in seconds
from the time the RS first sees it. Used to implement a weaker
from of token expiration for devices that cannot synchronize their
internal clocks.
o Change Controller: IESG
o Reference: Section 5.8.3 of [this document]
o Claim Name: "cnonce"
o Claim Description: "client-nonce". A nonce previously provided to
the AS by the RS via the client. Used verify token freshness when
the RS cannot synchronize its clock with the AS.
o Change Controller: IESG
o Reference: Section 5.8 of [this document]
8.13. CBOR Web Token Claims
This specification registers the following new claims in the "CBOR
Web Token (CWT) Claims" registry [IANA.CborWebTokenClaims].
o Claim Name: "scope"
o Claim Description: The scope of an access token as defined in
[RFC6749].
o JWT Claim Name: scope
o Claim Key: TBD (suggested: 9)
o Claim Value Type(s): byte string or text string
o Change Controller: IESG
o Specification Document(s): Section 5.8 of [this document]
o Claim Name: "profile"
o Claim Description: The profile a token is supposed to be used
with.
o JWT Claim Name: profile
o Claim Key: TBD (suggested: 38)
o Claim Value Type(s): integer
o Change Controller: IESG
o Specification Document(s): Section 5.8 of [this document]
o Claim Name: "exi"
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o Claim Description: The expiration time of a token measured from
when it was received at the RS in seconds.
o JWT Claim Name: exi
o Claim Key: TBD (suggested: 40)
o Claim Value Type(s): integer
o Change Controller: IESG
o Specification Document(s): Section 5.8.3 of [this document]
o Claim Name: "cnonce"
o Claim Description: The client-nonce sent to the AS by the RS via
the client.
o JWT Claim Name: cnonce
o Claim Key: TBD (suggested: 39)
o Claim Value Type(s): byte string
o Change Controller: IESG
o Specification Document(s): Section 5.8 of [this document]
8.14. Media Type Registrations
This specification registers the 'application/ace+cbor' media type
for messages of the protocols defined in this document carrying
parameters encoded in CBOR. This registration follows the procedures
specified in [RFC6838].
Type name: application
Subtype name: ace+cbor
Required parameters: none
Optional parameters: none
Encoding considerations: Must be encoded as CBOR map containing the
protocol parameters defined in [this document].
Security considerations: See Section 6 of this document.
Interoperability considerations: n/a
Published specification: [this document]
Applications that use this media type: The type is used by
authorization servers, clients and resource servers that support the
ACE framework as specified in [this document].
Additional information:
Magic number(s): n/a
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File extension(s): .ace
Macintosh file type code(s): n/a
Person & email address to contact for further information: Ludwig
Seitz <ludwig.seitz@ri.se>
Intended usage: COMMON
Restrictions on usage: None
Author: Ludwig Seitz <ludwig.setiz@ri.se>
Change controller: IESG
8.15. CoAP Content-Format Registry
This specification registers the following entry to the "CoAP
Content-Formats" registry:
Media Type: application/ace+cbor
Encoding
ID: 19
Reference: [this document]
8.16. Expert Review Instructions
All of the IANA registries established in this document are defined
as expert review. This section gives some general guidelines for
what the experts should be looking for, but they are being designated
as experts for a reason, so they should be given substantial
latitude.
Expert reviewers should take into consideration the following points:
o Point squatting should be discouraged. Reviewers are encouraged
to get sufficient information for registration requests to ensure
that the usage is not going to duplicate one that is already
registered, and that the point is likely to be used in
deployments. The zones tagged as private use are intended for
testing purposes and closed environments; code points in other
ranges should not be assigned for testing.
o Specifications are required for the standards track range of point
assignment. Specifications should exist for specification
required ranges, but early assignment before a specification is
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available is considered to be permissible. Specifications are
needed for the first-come, first-serve range if they are expected
to be used outside of closed environments in an interoperable way.
When specifications are not provided, the description provided
needs to have sufficient information to identify what the point is
being used for.
o Experts should take into account the expected usage of fields when
approving point assignment. The fact that there is a range for
standards track documents does not mean that a standards track
document cannot have points assigned outside of that range. The
length of the encoded value should be weighed against how many
code points of that length are left, the size of device it will be
used on, and the number of code points left that encode to that
size.
o Since a high degree of overlap is expected between these
registries and the contents of the OAuth parameters
[IANA.OAuthParameters] registries, experts should require new
registrations to maintain alignment with parameters from OAuth
that have comparable functionality. Deviation from this alignment
should only be allowed if there are functional differences, that
are motivated by the use case and that cannot be easily or
efficiently addressed by comparable OAuth parameters.
9. Acknowledgments
This document is a product of the ACE working group of the IETF.
Thanks to Eve Maler for her contributions to the use of OAuth 2.0 and
UMA in IoT scenarios, Robert Taylor for his discussion input, and
Malisa Vucinic for his input on the predecessors of this proposal.
Thanks to the authors of draft-ietf-oauth-pop-key-distribution, from
where large parts of the security considerations where copied.
Thanks to Stefanie Gerdes, Olaf Bergmann, and Carsten Bormann for
contributing their work on AS discovery from draft-gerdes-ace-dcaf-
authorize (see Section 5.1).
Thanks to Jim Schaad and Mike Jones for their comprehensive reviews.
Thanks to Benjamin Kaduk for his input on various questions related
to this work.
Thanks to Cigdem Sengul for some very useful review comments.
Ludwig Seitz and Goeran Selander worked on this document as part of
the CelticPlus project CyberWI, with funding from Vinnova. Ludwig
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Seitz was also received further funding for this work by Vinnova in
the context of the CelticNext project Critisec.
10. References
10.1. Normative References
[I-D.ietf-ace-cwt-proof-of-possession]
Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
Tschofenig, "Proof-of-Possession Key Semantics for CBOR
Web Tokens (CWTs)", draft-ietf-ace-cwt-proof-of-
possession-06 (work in progress), February 2019.
[I-D.ietf-ace-oauth-params]
Seitz, L., "Additional OAuth Parameters for Authorization
in Constrained Environments (ACE)", draft-ietf-ace-oauth-
params-04 (work in progress), February 2019.
[I-D.ietf-oauth-token-exchange]
Jones, M., Nadalin, A., Campbell, B., Bradley, J., and C.
Mortimore, "OAuth 2.0 Token Exchange", draft-ietf-oauth-
token-exchange-16 (work in progress), October 2018.
[IANA.CborWebTokenClaims]
IANA, "CBOR Web Token (CWT) Claims",
<https://www.iana.org/assignments/cwt/
cwt.xhtml#claims-registry>.
[IANA.JsonWebTokenClaims]
IANA, "JSON Web Token Claims",
<https://www.iana.org/assignments/jwt/jwt.xhtml#claims>.
[IANA.OAuthAccessTokenTypes]
IANA, "OAuth Access Token Types",
<https://www.iana.org/assignments/oauth-parameters/
oauth-parameters.xhtml#token-types>.
[IANA.OAuthParameters]
IANA, "OAuth Parameters",
<https://www.iana.org/assignments/oauth-parameters/
oauth-parameters.xhtml#parameters>.
[IANA.TokenIntrospectionResponse]
IANA, "OAuth Token Introspection Response",
<https://www.iana.org/assignments/oauth-parameters/
oauth-parameters.xhtml#token-introspection-response>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
Keranen, A., and P. Hallam-Baker, "Naming Things with
Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013,
<https://www.rfc-editor.org/info/rfc6920>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection",
RFC 7662, DOI 10.17487/RFC7662, October 2015,
<https://www.rfc-editor.org/info/rfc7662>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
10.2. Informative References
[I-D.erdtman-ace-rpcc]
Seitz, L. and S. Erdtman, "Raw-Public-Key and Pre-Shared-
Key as OAuth client credentials", draft-erdtman-ace-
rpcc-02 (work in progress), October 2017.
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-16 (work in
progress), March 2019.
[I-D.ietf-oauth-device-flow]
Denniss, W., Bradley, J., Jones, M., and H. Tschofenig,
"OAuth 2.0 Device Authorization Grant", draft-ietf-oauth-
device-flow-15 (work in progress), March 2019.
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-30 (work in progress),
November 2018.
[Margi10impact]
Margi, C., de Oliveira, B., de Sousa, G., Simplicio Jr,
M., Barreto, P., Carvalho, T., Naeslund, M., and R. Gold,
"Impact of Operating Systems on Wireless Sensor Networks
(Security) Applications and Testbeds", Proceedings of
the 19th International Conference on Computer
Communications and Networks (ICCCN), August 2010.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
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[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013,
<https://www.rfc-editor.org/info/rfc6819>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC7521] Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
"Assertion Framework for OAuth 2.0 Client Authentication
and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
May 2015, <https://www.rfc-editor.org/info/rfc7521>.
[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<https://www.rfc-editor.org/info/rfc7591>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC7744] Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M.,
and S. Kumar, "Use Cases for Authentication and
Authorization in Constrained Environments", RFC 7744,
DOI 10.17487/RFC7744, January 2016,
<https://www.rfc-editor.org/info/rfc7744>.
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[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8252] Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
<https://www.rfc-editor.org/info/rfc8252>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414,
DOI 10.17487/RFC8414, June 2018,
<https://www.rfc-editor.org/info/rfc8414>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8516] Keranen, A., ""Too Many Requests" Response Code for the
Constrained Application Protocol", RFC 8516,
DOI 10.17487/RFC8516, January 2019,
<https://www.rfc-editor.org/info/rfc8516>.
Appendix A. Design Justification
This section provides further insight into the design decisions of
the solution documented in this document. Section 3 lists several
building blocks and briefly summarizes their importance. The
justification for offering some of those building blocks, as opposed
to using OAuth 2.0 as is, is given below.
Common IoT constraints are:
Low Power Radio:
Many IoT devices are equipped with a small battery which needs to
last for a long time. For many constrained wireless devices, the
highest energy cost is associated to transmitting or receiving
messages (roughly by a factor of 10 compared to AES)
[Margi10impact]. It is therefore important to keep the total
communication overhead low, including minimizing the number and
size of messages sent and received, which has an impact of choice
on the message format and protocol. By using CoAP over UDP and
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CBOR encoded messages, some of these aspects are addressed.
Security protocols contribute to the communication overhead and
can, in some cases, be optimized. For example, authentication and
key establishment may, in certain cases where security
requirements allow, be replaced by provisioning of security
context by a trusted third party, using transport or application
layer security.
Low CPU Speed:
Some IoT devices are equipped with processors that are
significantly slower than those found in most current devices on
the Internet. This typically has implications on what timely
cryptographic operations a device is capable of performing, which
in turn impacts, e.g., protocol latency. Symmetric key
cryptography may be used instead of the computationally more
expensive public key cryptography where the security requirements
so allows, but this may also require support for trusted third
party assisted secret key establishment using transport or
application layer security.
Small Amount of Memory:
Microcontrollers embedded in IoT devices are often equipped with
small amount of RAM and flash memory, which places limitations
what kind of processing can be performed and how much code can be
put on those devices. To reduce code size fewer and smaller
protocol implementations can be put on the firmware of such a
device. In this case, CoAP may be used instead of HTTP, symmetric
key cryptography instead of public key cryptography, and CBOR
instead of JSON. Authentication and key establishment protocol,
e.g., the DTLS handshake, in comparison with assisted key
establishment also has an impact on memory and code.
User Interface Limitations:
Protecting access to resources is both an important security as
well as privacy feature. End users and enterprise customers may
not want to give access to the data collected by their IoT device
or to functions it may offer to third parties. Since the
classical approach of requesting permissions from end users via a
rich user interface does not work in many IoT deployment
scenarios, these functions need to be delegated to user-controlled
devices that are better suitable for such tasks, such as smart
phones and tablets.
Communication Constraints:
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In certain constrained settings an IoT device may not be able to
communicate with a given device at all times. Devices may be
sleeping, or just disconnected from the Internet because of
general lack of connectivity in the area, for cost reasons, or for
security reasons, e.g., to avoid an entry point for Denial-of-
Service attacks.
The communication interactions this framework builds upon (as
shown graphically in Figure 1) may be accomplished using a variety
of different protocols, and not all parts of the message flow are
used in all applications due to the communication constraints.
Deployments making use of CoAP are expected, but not limited to,
other protocols such as HTTP, HTTP/2 or other specific protocols,
such as Bluetooth Smart communication, that do not necessarily use
IP could also be used. The latter raises the need for application
layer security over the various interfaces.
In the light of these constraints we have made the following design
decisions:
CBOR, COSE, CWT:
This framework RECOMMENDS the use of CBOR [RFC7049] as data
format. Where CBOR data needs to be protected, the use of COSE
[RFC8152] is RECOMMENDED. Furthermore where self-contained tokens
are needed, this framework RECOMMENDS the use of CWT [RFC8392].
These measures aim at reducing the size of messages sent over the
wire, the RAM size of data objects that need to be kept in memory
and the size of libraries that devices need to support.
CoAP:
This framework RECOMMENDS the use of CoAP [RFC7252] instead of
HTTP. This does not preclude the use of other protocols
specifically aimed at constrained devices, like, e.g., Bluetooth
Low Energy (see Section 3.2). This aims again at reducing the
size of messages sent over the wire, the RAM size of data objects
that need to be kept in memory and the size of libraries that
devices need to support.
Access Information:
This framework defines the name "Access Information" for data
concerning the RS that the AS returns to the client in an access
token response (see Section 5.6.2). This aims at enabling
scenarios, where a powerful client, supporting multiple profiles,
needs to interact with a RS for which it does not know the
supported profiles and the raw public key.
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Proof-of-Possession:
This framework makes use of proof-of-possession tokens, using the
"cnf" claim [I-D.ietf-ace-cwt-proof-of-possession]. A
semantically and syntactically identical request and response
parameter is defined for the token endpoint, to allow requesting
and stating confirmation keys. This aims at making token theft
harder. Token theft is specifically relevant in constrained use
cases, as communication often passes through middle-boxes, which
could be able to steal bearer tokens and use them to gain
unauthorized access.
Auth-Info endpoint:
This framework introduces a new way of providing access tokens to
a RS by exposing a authz-info endpoint, to which access tokens can
be POSTed. This aims at reducing the size of the request message
and the code complexity at the RS. The size of the request
message is problematic, since many constrained protocols have
severe message size limitations at the physical layer (e.g., in
the order of 100 bytes). This means that larger packets get
fragmented, which in turn combines badly with the high rate of
packet loss, and the need to retransmit the whole message if one
packet gets lost. Thus separating sending of the request and
sending of the access tokens helps to reduce fragmentation.
Client Credentials Grant:
This framework RECOMMENDS the use of the client credentials grant
for machine-to-machine communication use cases, where manual
intervention of the resource owner to produce a grant token is not
feasible. The intention is that the resource owner would instead
pre-arrange authorization with the AS, based on the client's own
credentials. The client can then (without manual intervention)
obtain access tokens from the AS.
Introspection:
This framework RECOMMENDS the use of access token introspection in
cases where the client is constrained in a way that it can not
easily obtain new access tokens (i.e. it has connectivity issues
that prevent it from communicating with the AS). In that case
this framework RECOMMENDS the use of a long-term token, that could
be a simple reference. The RS is assumed to be able to
communicate with the AS, and can therefore perform introspection,
in order to learn the claims associated with the token reference.
The advantage of such an approach is that the resource owner can
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change the claims associated to the token reference without having
to be in contact with the client, thus granting or revoking access
rights.
Appendix B. Roles and Responsibilities
Resource Owner
* Make sure that the RS is registered at the AS. This includes
making known to the AS which profiles, token_types, scopes, and
key types (symmetric/asymmetric) the RS supports. Also making
it known to the AS which audience(s) the RS identifies itself
with.
* Make sure that clients can discover the AS that is in charge of
the RS.
* If the client-credentials grant is used, make sure that the AS
has the necessary, up-to-date, access control policies for the
RS.
Requesting Party
* Make sure that the client is provisioned the necessary
credentials to authenticate to the AS.
* Make sure that the client is configured to follow the security
requirements of the Requesting Party when issuing requests
(e.g., minimum communication security requirements, trust
anchors).
* Register the client at the AS. This includes making known to
the AS which profiles, token_types, and key types (symmetric/
asymmetric) the client.
Authorization Server
* Register the RS and manage corresponding security contexts.
* Register clients and authentication credentials.
* Allow Resource Owners to configure and update access control
policies related to their registered RSs.
* Expose the token endpoint to allow clients to request tokens.
* Authenticate clients that wish to request a token.
* Process a token request using the authorization policies
configured for the RS.
* Optionally: Expose the introspection endpoint that allows RS's
to submit token introspection requests.
* If providing an introspection endpoint: Authenticate RSs that
wish to get an introspection response.
* If providing an introspection endpoint: Process token
introspection requests.
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* Optionally: Handle token revocation.
* Optionally: Provide discovery metadata. See [RFC8414]
* Optionally: Handle refresh tokens.
Client
* Discover the AS in charge of the RS that is to be targeted with
a request.
* Submit the token request (see step (A) of Figure 1).
+ Authenticate to the AS.
+ Optionally (if not pre-configured): Specify which RS, which
resource(s), and which action(s) the request(s) will target.
+ If raw public keys (rpk) or certificates are used, make sure
the AS has the right rpk or certificate for this client.
* Process the access token and Access Information (see step (B)
of Figure 1).
+ Check that the Access Information provides the necessary
security parameters (e.g., PoP key, information on
communication security protocols supported by the RS).
+ Safely store the proof-of-possession key.
+ If provided by the AS: Safely store the refresh token.
* Send the token and request to the RS (see step (C) of
Figure 1).
+ Authenticate towards the RS (this could coincide with the
proof of possession process).
+ Transmit the token as specified by the AS (default is to the
authz-info endpoint, alternative options are specified by
profiles).
+ Perform the proof-of-possession procedure as specified by
the profile in use (this may already have been taken care of
through the authentication procedure).
* Process the RS response (see step (F) of Figure 1) of the RS.
Resource Server
* Expose a way to submit access tokens. By default this is the
authz-info endpoint.
* Process an access token.
+ Verify the token is from a recognized AS.
+ Verify that the token applies to this RS.
+ Check that the token has not expired (if the token provides
expiration information).
+ Check the token's integrity.
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+ Store the token so that it can be retrieved in the context
of a matching request.
* Process a request.
+ Set up communication security with the client.
+ Authenticate the client.
+ Match the client against existing tokens.
+ Check that tokens belonging to the client actually authorize
the requested action.
+ Optionally: Check that the matching tokens are still valid,
using introspection (if this is possible.)
* Send a response following the agreed upon communication
security.
* Safely store credentials such as raw public keys for
authentication or proof-of-possession keys linked to access
tokens.
Appendix C. Requirements on Profiles
This section lists the requirements on profiles of this framework,
for the convenience of profile designers.
o Specify the communication protocol the client and RS the must use
(e.g., CoAP). Section 5 and Section 5.6.4.3
o Specify the security protocol the client and RS must use to
protect their communication (e.g., OSCORE or DTLS over CoAP).
This must provide encryption, integrity and replay protection.
Section 5.6.4.3
o Specify how the client and the RS mutually authenticate.
Section 4
o Specify the proof-of-possession protocol(s) and how to select one,
if several are available. Also specify which key types (e.g.,
symmetric/asymmetric) are supported by a specific proof-of-
possession protocol. Section 5.6.4.2
o Specify a unique profile identifier. Section 5.6.4.3
o If introspection is supported: Specify the communication and
security protocol for introspection. Section 5.7
o Specify the communication and security protocol for interactions
between client and AS. This must provide encryption, integrity
protection, replay protection and a binding between requests and
responses. Section 5 and Section 5.6
o Specify how/if the authz-info endpoint is protected, including how
error responses are protected. Section 5.8.1
o Optionally define other methods of token transport than the authz-
info endpoint. Section 5.8.1
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Appendix D. Assumptions on AS knowledge about C and RS
This section lists the assumptions on what an AS should know about a
client and a RS in order to be able to respond to requests to the
token and introspection endpoints. How this information is
established is out of scope for this document.
o The identifier of the client or RS.
o The profiles that the client or RS supports.
o The scopes that the RS supports.
o The audiences that the RS identifies with.
o The key types (e.g., pre-shared symmetric key, raw public key, key
length, other key parameters) that the client or RS supports.
o The types of access tokens the RS supports (e.g., CWT).
o If the RS supports CWTs, the COSE parameters for the crypto
wrapper (e.g., algorithm, key-wrap algorithm, key-length).
o The expiration time for access tokens issued to this RS (unless
the RS accepts a default time chosen by the AS).
o The symmetric key shared between client or RS and AS (if any).
o The raw public key of the client or RS (if any).
o Whether the RS has synchronized time (and thus is able to use the
'exp' claim) or not.
Appendix E. Deployment Examples
There is a large variety of IoT deployments, as is indicated in
Appendix A, and this section highlights a few common variants. This
section is not normative but illustrates how the framework can be
applied.
For each of the deployment variants, there are a number of possible
security setups between clients, resource servers and authorization
servers. The main focus in the following subsections is on how
authorization of a client request for a resource hosted by a RS is
performed. This requires the security of the requests and responses
between the clients and the RS to consider.
Note: CBOR diagnostic notation is used for examples of requests and
responses.
E.1. Local Token Validation
In this scenario, the case where the resource server is offline is
considered, i.e., it is not connected to the AS at the time of the
access request. This access procedure involves steps A, B, C, and F
of Figure 1.
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Since the resource server must be able to verify the access token
locally, self-contained access tokens must be used.
This example shows the interactions between a client, the
authorization server and a temperature sensor acting as a resource
server. Message exchanges A and B are shown in Figure 17.
A: The client first generates a public-private key pair used for
communication security with the RS.
The client sends the POST request to the token endpoint at the AS.
The security of this request can be transport or application
layer. It is up the the communication security profile to define.
In the example transport layer identification of the AS is done
and the client identifies with client_id and client_secret as in
classic OAuth. The request contains the public key of the client
and the Audience parameter set to "tempSensorInLivingRoom", a
value that the temperature sensor identifies itself with. The AS
evaluates the request and authorizes the client to access the
resource.
B: The AS responds with a PoP access token and Access Information.
The PoP access token contains the public key of the client, and
the Access Information contains the public key of the RS. For
communication security this example uses DTLS RawPublicKey between
the client and the RS. The issued token will have a short
validity time, i.e., "exp" close to "iat", to protect the RS from
replay attacks. The token includes the claim such as "scope" with
the authorized access that an owner of the temperature device can
enjoy. In this example, the "scope" claim, issued by the AS,
informs the RS that the owner of the token, that can prove the
possession of a key is authorized to make a GET request against
the /temperature resource and a POST request on the /firmware
resource. Note that the syntax and semantics of the scope claim
are application specific.
Note: In this example it is assumed that the client knows what
resource it wants to access, and is therefore able to request
specific audience and scope claims for the access token.
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Authorization
Client Server
| |
|<=======>| DTLS Connection Establishment
| | to identify the AS
| |
A: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"token"
| | Content-Format: application/ace+cbor
| | Payload: <Request-Payload>
| |
B: |<--------+ Header: 2.05 Content
| 2.05 | Content-Format: application/ace+cbor
| | Payload: <Response-Payload>
| |
Figure 17: Token Request and Response Using Client Credentials.
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 18 Note that the parameter "rs_cnf" from
[I-D.ietf-ace-oauth-params] is used to inform the client about the
resource server's public key.
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Request-Payload :
{
"audience" : "tempSensorInLivingRoom",
"client_id" : "myclient",
"client_secret" : "qwerty"
"req_cnf" : {
"COSE_Key" : {
"kid" : b64'1Bg8vub9tLe1gHMzV76e8',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
"y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
}
}
}
Response-Payload :
{
"access_token" : b64'SlAV32hkKG ...',
"rs_cnf" : {
"COSE_Key" : {
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4',
"y" : b64'4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM'
}
}
}
Figure 18: Request and Response Payload Details.
The content of the access token is shown in Figure 19.
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{
"aud" : "tempSensorInLivingRoom",
"iat" : "1360189224",
"exp" : "1360289224",
"scope" : "temperature_g firmware_p",
"cnf" : {
"COSE_Key" : {
"kid" : b64'1Bg8vub9tLe1gHMzV76e8',
"kty" : "EC",
"crv" : "P-256",
"x" : b64'f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU',
"y" : b64'x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0'
}
}
}
Figure 19: Access Token including Public Key of the Client.
Messages C and F are shown in Figure 20 - Figure 21.
C: The client then sends the PoP access token to the authz-info
endpoint at the RS. This is a plain CoAP request, i.e., no
transport or application layer security is used between client and
RS since the token is integrity protected between the AS and RS.
The RS verifies that the PoP access token was created by a known
and trusted AS, is valid, and has been issued to the client. The
RS caches the security context together with authorization
information about this client contained in the PoP access token.
Resource
Client Server
| |
C: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"authz-info"
| | Payload: SlAV32hkKG ...
| |
|<--------+ Header: 2.04 Changed
| 2.04 |
| |
Figure 20: Access Token provisioning to RS
The client and the RS runs the DTLS handshake using the raw public
keys established in step B and C.
The client sends the CoAP request GET to /temperature on RS over
DTLS. The RS verifies that the request is authorized, based on
previously established security context.
F: The RS responds with a resource representation over DTLS.
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Resource
Client Server
| |
|<=======>| DTLS Connection Establishment
| | using Raw Public Keys
| |
+-------->| Header: GET (Code=0.01)
| GET | Uri-Path: "temperature"
| |
| |
| |
F: |<--------+ Header: 2.05 Content
| 2.05 | Payload: <sensor value>
| |
Figure 21: Resource Request and Response protected by DTLS.
E.2. Introspection Aided Token Validation
In this deployment scenario it is assumed that a client is not able
to access the AS at the time of the access request, whereas the RS is
assumed to be connected to the back-end infrastructure. Thus the RS
can make use of token introspection. This access procedure involves
steps A-F of Figure 1, but assumes steps A and B have been carried
out during a phase when the client had connectivity to AS.
Since the client is assumed to be offline, at least for a certain
period of time, a pre-provisioned access token has to be long-lived.
Since the client is constrained, the token will not be self contained
(i.e. not a CWT) but instead just a reference. The resource server
uses its connectivity to learn about the claims associated to the
access token by using introspection, which is shown in the example
below.
In the example interactions between an offline client (key fob), a RS
(online lock), and an AS is shown. It is assumed that there is a
provisioning step where the client has access to the AS. This
corresponds to message exchanges A and B which are shown in
Figure 22.
Authorization consent from the resource owner can be pre-configured,
but it can also be provided via an interactive flow with the resource
owner. An example of this for the key fob case could be that the
resource owner has a connected car, he buys a generic key that he
wants to use with the car. To authorize the key fob he connects it
to his computer that then provides the UI for the device. After that
OAuth 2.0 implicit flow can used to authorize the key for his car at
the the car manufacturers AS.
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Note: In this example the client does not know the exact door it will
be used to access since the token request is not send at the time of
access. So the scope and audience parameters are set quite wide to
start with and new values different form the original once can be
returned from introspection later on.
A: The client sends the request using POST to the token endpoint
at AS. The request contains the Audience parameter set to
"PACS1337" (PACS, Physical Access System), a value the that the
online door in question identifies itself with. The AS generates
an access token as an opaque string, which it can match to the
specific client, a targeted audience and a symmetric key. The
security is provided by identifying the AS on transport layer
using a pre shared security context (psk, rpk or certificate) and
then the client is identified using client_id and client_secret as
in classic OAuth.
B: The AS responds with the an access token and Access
Information, the latter containing a symmetric key. Communication
security between C and RS will be DTLS and PreSharedKey. The PoP
key is used as the PreSharedKey.
Authorization
Client Server
| |
| |
A: +-------->| Header: POST (Code=0.02)
| POST | Uri-Path:"token"
| | Content-Format: application/ace+cbor
| | Payload: <Request-Payload>
| |
B: |<--------+ Header: 2.05 Content
| | Content-Format: application/ace+cbor
| 2.05 | Payload: <Response-Payload>
| |
Figure 22: Token Request and Response using Client Credentials.
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 23.
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Request-Payload:
{
"client_id" : "keyfob",
"client_secret" : "qwerty"
}
Response-Payload:
{
"access_token" : b64'VGVzdCB0b2tlbg==',
"cnf" : {
"COSE_Key" : {
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk',
"kty" : "oct",
"alg" : "HS256",
"k": b64'ZoRSOrFzN_FzUA5XKMYoVHyzff5oRJxl-IXRtztJ6uE'
}
}
}
Figure 23: Request and Response Payload for C offline
The access token in this case is just an opaque byte string
referencing the authorization information at the AS.
C: Next, the client POSTs the access token to the authz-info
endpoint in the RS. This is a plain CoAP request, i.e., no DTLS
between client and RS. Since the token is an opaque string, the
RS cannot verify it on its own, and thus defers to respond the
client with a status code until after step E.
D: The RS forwards the token to the introspection endpoint on the
AS. Introspection assumes a secure connection between the AS and
the RS, e.g., using transport of application layer security. In
the example AS is identified using pre shared security context
(psk, rpk or certificate) while RS is acting as client and is
identified with client_id and client_secret.
E: The AS provides the introspection response containing
parameters about the token. This includes the confirmation key
(cnf) parameter that allows the RS to verify the client's proof of
possession in step F.
After receiving message E, the RS responds to the client's POST in
step C with the CoAP response code 2.01 (Created).
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Resource
Client Server
| |
C: +-------->| Header: POST (T=CON, Code=0.02)
| POST | Uri-Path:"authz-info"
| | Payload: b64'VGVzdCB0b2tlbg=='
| |
| | Authorization
| | Server
| | |
| D: +--------->| Header: POST (Code=0.02)
| | POST | Uri-Path: "introspect"
| | | Content-Format: "application/ace+cbor"
| | | Payload: <Request-Payload>
| | |
| E: |<---------+ Header: 2.05 Content
| | 2.05 | Content-Format: "application/ace+cbor"
| | | Payload: <Response-Payload>
| | |
| |
|<--------+ Header: 2.01 Created
| 2.01 |
| |
Figure 24: Token Introspection for C offline
The information contained in the Request-Payload and the Response-
Payload is shown in Figure 25.
Request-Payload:
{
"token" : b64'VGVzdCB0b2tlbg==',
"client_id" : "FrontDoor",
"client_secret" : "ytrewq"
}
Response-Payload:
{
"active" : true,
"aud" : "lockOfDoor4711",
"scope" : "open, close",
"iat" : 1311280970,
"cnf" : {
"kid" : b64'c29tZSBwdWJsaWMga2V5IGlk'
}
}
Figure 25: Request and Response Payload for Introspection
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The client uses the symmetric PoP key to establish a DTLS
PreSharedKey secure connection to the RS. The CoAP request PUT is
sent to the uri-path /state on the RS, changing the state of the
door to locked.
F: The RS responds with a appropriate over the secure DTLS
channel.
Resource
Client Server
| |
|<=======>| DTLS Connection Establishment
| | using Pre Shared Key
| |
+-------->| Header: PUT (Code=0.03)
| PUT | Uri-Path: "state"
| | Payload: <new state for the lock>
| |
F: |<--------+ Header: 2.04 Changed
| 2.04 | Payload: <new state for the lock>
| |
Figure 26: Resource request and response protected by OSCORE
Appendix F. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
F.1. Version -21 to 22
o Provided section numbers in references to OAuth RFC.
o Updated IANA mapping registries to only use "Private Use" and
"Expert Review".
o Made error messages optional for RS at token submission since it
may not be able to send them depending on the profile.
o Corrected errors in examples.
F.2. Version -20 to 21
o Added text about expiration of RS keys.
F.3. Version -19 to 20
o Replaced "req_aud" with "audience" from the OAuth token exchange
draft.
o Updated examples to remove unnecessary elements.
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F.4. Version -18 to -19
o Added definition of "Authorization Information".
o Explicitly state that ACE allows encoding refresh tokens in binary
format in addition to strings.
o Renamed "AS Information" to "AS Request Creation Hints" and added
the possibility to specify req_aud and scope as hints.
o Added the "kid" parameter to AS Request Creation Hints.
o Added security considerations about the integrity protection of
tokens with multi-RS audiences.
o Renamed IANA registries mapping OAuth parameters to reflect the
mapped registry.
o Added JWT claim names to CWT claim registrations.
o Added expert review instructions.
o Updated references to TLS from 1.2 to 1.3.
F.5. Version -17 to -18
o Added OSCORE options in examples involving OSCORE.
o Removed requirement for the client to send application/cwt, since
the client has no way to know.
o Clarified verification of tokens by the RS.
o Added exi claim CWT registration.
F.6. Version -16 to -17
o Added references to (D)TLS 1.3.
o Added requirement that responses are bound to requests.
o Specify that grant_type is OPTIONAL in C2AS requests (as opposed
to REQUIRED in OAuth).
o Replaced examples with hypothetical COSE profile with OSCORE.
o Added requirement for content type application/ace+cbor in error
responses for token and introspection requests and responses.
o Reworked abbreviation space for claims, request and response
parameters.
o Added text that the RS may indicate that it is busy at the authz-
info resource.
o Added section that specifies how the RS verifies an access token.
o Added section on the protection of the authz-info endpoint.
o Removed the expiration mechanism based on sequence numbers.
o Added reference to RFC7662 security considerations.
o Added considerations on minimal security requirements for
communication.
o Added security considerations on unprotected information sent to
authz-info and in the error responses.
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F.7. Version -15 to -16
o Added text the RS using RFC6750 error codes.
o Defined an error code for incompatible token request parameters.
o Removed references to the actors draft.
o Fixed errors in examples.
F.8. Version -14 to -15
o Added text about refresh tokens.
o Added text about protection of credentials.
o Rephrased introspection so that other entities than RS can do it.
o Editorial improvements.
F.9. Version -13 to -14
o Split out the 'aud', 'cnf' and 'rs_cnf' parameters to
[I-D.ietf-ace-oauth-params]
o Introduced the "application/ace+cbor" Content-Type.
o Added claim registrations from 'profile' and 'rs_cnf'.
o Added note on schema part of AS Information Section 5.1.2
o Realigned the parameter abbreviations to push rarely used ones to
the 2-byte encoding size of CBOR integers.
F.10. Version -12 to -13
o Changed "Resource Information" to "Access Information" to avoid
confusion.
o Clarified section about AS discovery.
o Editorial changes
F.11. Version -11 to -12
o Moved the Request error handling to a section of its own.
o Require the use of the abbreviation for profile identifiers.
o Added rs_cnf parameter in the introspection response, to inform
RS' with several RPKs on which key to use.
o Allowed use of rs_cnf as claim in the access token in order to
inform an RS with several RPKs on which key to use.
o Clarified that profiles must specify if/how error responses are
protected.
o Fixed label number range to align with COSE/CWT.
o Clarified the requirements language in order to allow profiles to
specify other payload formats than CBOR if they do not use CoAP.
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F.12. Version -10 to -11
o Fixed some CBOR data type errors.
o Updated boilerplate text
F.13. Version -09 to -10
o Removed CBOR major type numbers.
o Removed the client token design.
o Rephrased to clarify that other protocols than CoAP can be used.
o Clarifications regarding the use of HTTP
F.14. Version -08 to -09
o Allowed scope to be byte strings.
o Defined default names for endpoints.
o Refactored the IANA section for briefness and consistency.
o Refactored tables that define IANA registry contents for
consistency.
o Created IANA registry for CBOR mappings of error codes, grant
types and Authorization Server Information.
o Added references to other document sections defining IANA entries
in the IANA section.
F.15. Version -07 to -08
o Moved AS discovery from the DTLS profile to the framework, see
Section 5.1.
o Made the use of CBOR mandatory. If you use JSON you can use
vanilla OAuth.
o Made it mandatory for profiles to specify C-AS security and RS-AS
security (the latter only if introspection is supported).
o Made the use of CBOR abbreviations mandatory.
o Added text to clarify the use of token references as an
alternative to CWTs.
o Added text to clarify that introspection must not be delayed, in
case the RS has to return a client token.
o Added security considerations about leakage through unprotected AS
discovery information, combining profiles and leakage through
error responses.
o Added privacy considerations about leakage through unprotected AS
discovery.
o Added text that clarifies that introspection is optional.
o Made profile parameter optional since it can be implicit.
o Clarified that CoAP is not mandatory and other protocols can be
used.
o Clarified the design justification for specific features of the
framework in appendix A.
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o Clarified appendix E.2.
o Removed specification of the "cnf" claim for CBOR/COSE, and
replaced with references to [I-D.ietf-ace-cwt-proof-of-possession]
F.16. Version -06 to -07
o Various clarifications added.
o Fixed erroneous author email.
F.17. Version -05 to -06
o Moved sections that define the ACE framework into a subsection of
the framework Section 5.
o Split section on client credentials and grant into two separate
sections, Section 5.2, and Section 5.3.
o Added Section 5.4 on AS authentication.
o Added Section 5.5 on the Authorization endpoint.
F.18. Version -04 to -05
o Added RFC 2119 language to the specification of the required
behavior of profile specifications.
o Added Section 5.3 on the relation to the OAuth2 grant types.
o Added CBOR abbreviations for error and the error codes defined in
OAuth2.
o Added clarification about token expiration and long-running
requests in Section 5.8.3
o Added security considerations about tokens with symmetric pop keys
valid for more than one RS.
o Added privacy considerations section.
o Added IANA registry mapping the confirmation types from RFC 7800
to equivalent COSE types.
o Added appendix D, describing assumptions about what the AS knows
about the client and the RS.
F.19. Version -03 to -04
o Added a description of the terms "framework" and "profiles" as
used in this document.
o Clarified protection of access tokens in section 3.1.
o Clarified uses of the "cnf" parameter in section 6.4.5.
o Clarified intended use of Client Token in section 7.4.
F.20. Version -02 to -03
o Removed references to draft-ietf-oauth-pop-key-distribution since
the status of this draft is unclear.
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o Copied and adapted security considerations from draft-ietf-oauth-
pop-key-distribution.
o Renamed "client information" to "RS information" since it is
information about the RS.
o Clarified the requirements on profiles of this framework.
o Clarified the token endpoint protocol and removed negotiation of
"profile" and "alg" (section 6).
o Renumbered the abbreviations for claims and parameters to get a
consistent numbering across different endpoints.
o Clarified the introspection endpoint.
o Renamed token, introspection and authz-info to "endpoint" instead
of "resource" to mirror the OAuth 2.0 terminology.
o Updated the examples in the appendices.
F.21. Version -01 to -02
o Restructured to remove communication security parts. These shall
now be defined in profiles.
o Restructured section 5 to create new sections on the OAuth
endpoints token, introspection and authz-info.
o Pulled in material from draft-ietf-oauth-pop-key-distribution in
order to define proof-of-possession key distribution.
o Introduced the "cnf" parameter as defined in RFC7800 to reference
or transport keys used for proof of possession.
o Introduced the "client-token" to transport client information from
the AS to the client via the RS in conjunction with introspection.
o Expanded the IANA section to define parameters for token request,
introspection and CWT claims.
o Moved deployment scenarios to the appendix as examples.
F.22. Version -00 to -01
o Changed 5.1. from "Communication Security Protocol" to "Client
Information".
o Major rewrite of 5.1 to clarify the information exchanged between
C and AS in the PoP access token request profile for IoT.
* Allow the client to indicate preferences for the communication
security protocol.
* Defined the term "Client Information" for the additional
information returned to the client in addition to the access
token.
* Require that the messages between AS and client are secured,
either with (D)TLS or with COSE_Encrypted wrappers.
* Removed dependency on OSCOAP and added generic text about
object security instead.
* Defined the "rpk" parameter in the client information to
transmit the raw public key of the RS from AS to client.
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* (D)TLS MUST use the PoP key in the handshake (either as PSK or
as client RPK with client authentication).
* Defined the use of x5c, x5t and x5tS256 parameters when a
client certificate is used for proof of possession.
* Defined "tktn" parameter for signaling for how to transfer the
access token.
o Added 5.2. the CoAP Access-Token option for transferring access
tokens in messages that do not have payload.
o 5.3.2. Defined success and error responses from the RS when
receiving an access token.
o 5.6.:Added section giving guidance on how to handle token
expiration in the absence of reliable time.
o Appendix B Added list of roles and responsibilities for C, AS and
RS.
Authors' Addresses
Ludwig Seitz
RISE
Scheelevaegen 17
Lund 223 70
Sweden
Email: ludwig.seitz@ri.se
Goeran Selander
Ericsson
Faroegatan 6
Kista 164 80
Sweden
Email: goran.selander@ericsson.com
Erik Wahlstroem
Sweden
Email: erik@wahlstromstekniska.se
Samuel Erdtman
Spotify AB
Birger Jarlsgatan 61, 4tr
Stockholm 113 56
Sweden
Email: erdtman@spotify.com
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Hannes Tschofenig
Arm Ltd.
Absam 6067
Austria
Email: Hannes.Tschofenig@arm.com
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